Add support for AWS SES Global Endpoints (Multi-Region Endpoints) which
provide multi-region high availability and automatic failover between
two AWS regions.

Changes:
- Add AWS_SES_GLOBAL_ENDPOINT_ID setting to configure endpoint ID
- Implement EndpointId parameter in send_email API calls (SESv2 only)
- Add validation to ensure SESv2 is enabled when using global endpoints
- Add comprehensive test coverage for global endpoint functionality
- Update documentation with setup instructions and usage examples

The implementation:
- Maintains full backward compatibility (default: None)
- Requires USE_SES_V2=True (validated at runtime)
- Follows existing code patterns and project conventions
- Includes 4 new test classes covering all scenarios

Package operations: 0 installs, 12 updates, 0 removals

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