feat(envelopes): add EventBridge envelope

[j-d-ha]

Description

Add support for EventBridge events using the EventBridge envelope.

Tasks

• Create package: AwsLambda.Host.Envelopes.EventBridge
• Implement EventBridgeEnvelope<T> class
• Implement IRequestEnvelope interface
• Create comprehensive README.md
• Add unit tests in tests/AwsLambda.Host.Envelopes.UnitTests/
• Update solution file
• Commit with conventional format

Acceptance Criteria

• Detail field extraction from EventBridge events working
• Support for various EventBridge event sources
• Full unit test coverage
• README includes examples and AOT guidance
• Package builds for all target frameworks