Add CloudWatch EMF exporter foundation with basic Gauge support

This PR introduces the core structure for the CloudWatch EMF (Embedded Metric Format)
exporter for OpenTelemetry Python. This foundation includes:

Core Features:
- CloudWatchEMFExporter class with full initialization
- Basic EMF log structure creation
- Gauge metric conversion and export
- Unit mapping from OTel to CloudWatch units
- CloudWatch Logs integration with log group/stream management
- Metric grouping by attributes and timestamps
- Basic error handling and logging

Architecture:
- Follows OTel MetricExporter interface
- Uses boto3 for CloudWatch Logs integration
- Implements proper resource attribute handling
- Supports DELTA temporality for CloudWatch compatibility

Testing:
- Comprehensive unit tests for core functionality
- Mock-based testing to avoid AWS dependencies
- Tests for initialization, conversion, and basic export flow

Future PRs will add:
- Support for Sum, Histogram, and ExponentialHistogram metrics
- Advanced batching and CloudWatch Logs constraints
- Enhanced error handling and validation

Author: mxiamxia