feat(envelopes): add VPC Lattice envelope

[j-d-ha]

Description

Add support for VPC Lattice events using the VPC Lattice envelope.

Tasks

• Create package: AwsLambda.Host.Envelopes.VpcLattice
• Implement VpcLatticeRequestEnvelope<T> class
• Implement VpcLatticeResponseEnvelope<T> class
• Implement IRequestEnvelope and IResponseEnvelope interfaces
• Create comprehensive README.md
• Add unit tests in tests/AwsLambda.Host.Envelopes.UnitTests/
• Update solution file
• Commit with conventional format

Acceptance Criteria

• VPC Lattice request/response handling working
• Full unit test coverage
• README includes examples and AOT guidance
• Package builds for all target frameworks