feat(ElastiCache): add support for serverless cache.

[Ashish0804]

Fixes #676

Adds support for monitoring Serverless Elasticache.

Generated widget would look like:

┌─────────────────────────────────────────────────────────────┐
│  Title Widget: "Serverless ElastiCache - {cluster name}"   │                                                                                                                          
└─────────────────────────────────────────────────────────────┘                                                                                                                         
                                                                                                                                                                                        
┌──────────────────┬──────────────────────────────────────────┐                                                                                                                         
│  Cache Size      │  Hits/Misses                             │                                                                                                                         
│  (1/3 width)     │  (2/3 width)                             │                                                                                                                         
│                  │  Left: Hit Rate %                        │                                                                                                                         
│  Avg Cache Size  │  Right: Cache Hits/Misses Count          │                                                                                                                         
└──────────────────┴──────────────────────────────────────────┘                                                                                                                         
                                                                                                                                                                                        
┌──────────────────────────────┬──────────────────────────────┐                                                                                                                         
│  Successful Read Latency     │  Successful Write Latency    │                                                                                                                         
│  (1/2 width)                 │  (1/2 width)                 │                                                                                                                         
│                              │                              │                                                                                                                         
│  TM99 + Average Latency      │  TM99 + Average Latency      │                                                                                                                         
└──────────────────────────────┴──────────────────────────────┘                                                                                                                         
                                                                                                                                                                                        
┌──────────────────┬──────────────────────────────────────────┐                                                                                                                         
│  IO              │  Commands                                │                                                                                                                         
│  (1/3 width)     │  (2/3 width)                             │                                                                                                                         
│                  │  Left: Throttle Rate %                   │                                                                                                                         
│  Bytes In/Out    │  Right: Total Commands                   │                                                                                                                         
└──────────────────┴──────────────────────────────────────────┘                                                                                                                         
                                                                                                                                                                                        
┌──────────────────┬──────────────────┬──────────────────────┐                                                                                                                          
│  Processing Units│  Items           │  Connections         │                                                                                                                          
│  (1/3 width)     │  (1/3 width)     │  (1/3 width)         │                                                                                                                          
│                  │                  │                      │                                                                                                                          
│  Max ECPU        │  Item Count +    │  Avg Connections     │                                                                                                                          
│                  │  Evictions       │                      │                                                                                                                          
└──────────────────┴──────────────────┴──────────────────────┘                                                                                                                          
                                                                                                                                                                                        

Summary:
• **Row 1**: Title header
• **Row 2**: Cache size (1/3) + Hit/Miss rates (2/3)
• **Row 3**: Read latency (1/2) + Write latency (1/2)
• **Row 4**: Network I/O (1/3) + Commands/Throttling (2/3)
• **Row 5**: Processing units (1/3) + Item count (1/3) + Connections (1/3)

Each widget includes alarm annotations where configured, displayed as horizontal lines on the graphs.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[Ashish0804]

The build workflow is failing: https://github.com/Ashish0804/cdk-monitoring-constructs/actions/runs/19360798211

The failure was on my local mac aswell, but i assumed it was because of my setup.
I'll include that fix in the CR if needed.

[Ashish0804]

New build passes: https://github.com/Ashish0804/cdk-monitoring-constructs/actions/runs/19360957789