feat(envelopes): add DynamoDB Stream envelope

[j-d-ha]

Description

Add support for DynamoDB Stream events using the DynamoDB Stream envelope.

Tasks

• Create package: AwsLambda.Host.Envelopes.DynamoDbStream
• Implement DynamoDbStreamEnvelope<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

• DynamoDB stream records with NewImage/OldImage extraction working
• Full unit test coverage
• README includes examples and AOT guidance
• Package builds for all target frameworks (net8.0, net9.0, net10.0)