Add Multi-Source API Example (#598)

This PR adds a new example demonstrating how to build a Lambda function
that handles requests from multiple sources (Application Load Balancer
and API Gateway V2) using a single handler.

### What's New

**New Example: `Examples/MultiSourceAPI`**

A Lambda function that:
- Implements `StreamingLambdaHandler` to accept raw `ByteBuffer` events
- Dynamically decodes events as either `ALBTargetGroupRequest` or
`APIGatewayV2Request`
- Returns appropriate responses based on the detected event source
- Demonstrates handling multiple AWS service integrations with a single
function

### Key Features

- **Type-safe event detection**: Uses Swift's `Decodable` to identify
the event source at runtime
- **Streaming response**: Implements `StreamingLambdaHandler` for
efficient response handling
- **Complete deployment**: Includes SAM template with both ALB and API
Gateway V2 infrastructure
- **Production-ready**: Full VPC setup with subnets, security groups,
and load balancer configuration

### Files Added

- `Examples/MultiSourceAPI/Sources/main.swift` - Lambda handler
implementation
- `Examples/MultiSourceAPI/Package.swift` - Swift package configuration
- `Examples/MultiSourceAPI/template.yaml` - SAM deployment template with
ALB and API Gateway V2
- `Examples/MultiSourceAPI/README.md` - Documentation with build,
deploy, and test instructions
- Updated CI to include the new example

### Use Case

This pattern is useful when you want to:
- Expose the same Lambda function through multiple AWS services
- Reduce code duplication by handling similar requests from different
sources
- Maintain a single codebase for multi-channel APIs

---------

Co-authored-by: Sebastien Stormacq <stormacq@amazon.lu>
Co-authored-by: Josh Elkins <jbelkins@users.noreply.github.com>

Examples/MultiSourceAPI/.gitignore

@@ -0,0 +1,11 @@
+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc
+Package.resolved
+aws-sam
+samconfig.toml