Add HyperLogLog support

[imperugo]

Summary

Add HyperLogLog operations for probabilistic cardinality estimation (unique counts).

Use Cases

• Count unique visitors without storing all IDs
• Approximate distinct event counts at scale
• Merge counts from multiple sources

Proposed API

```csharp
Task HyperLogLogAddAsync(string key, T value, CommandFlags flag = CommandFlags.None);
Task HyperLogLogAddAsync(string key, T[] values, CommandFlags flag = CommandFlags.None);
Task HyperLogLogLengthAsync(string key, CommandFlags flag = CommandFlags.None);
Task HyperLogLogLengthAsync(string[] keys, CommandFlags flag = CommandFlags.None);
Task HyperLogLogMergeAsync(string destinationKey, string[] sourceKeys, CommandFlags flag = CommandFlags.None);
```

SE.Redis provides all native methods. Thin wrappers with serialization on Add.