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/Sources/main.swift

@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright SwiftAWSLambdaRuntime project authors
+// Copyright (c) Amazon.com, Inc. or its affiliates.
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftAWSLambdaRuntime project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import AWSLambdaEvents
+import AWSLambdaRuntime
+import NIOCore
+
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
+import Foundation
+#endif
+
+struct MultiSourceHandler: StreamingLambdaHandler {
+    func handle(
+        _ event: ByteBuffer,
+        responseWriter: some LambdaResponseStreamWriter,
+        context: LambdaContext
+    ) async throws {
+        let decoder = JSONDecoder()
+        let data = Data(event.readableBytesView)
+
+        // Try to decode as ALBTargetGroupRequest first
+        if let albRequest = try? decoder.decode(ALBTargetGroupRequest.self, from: data) {
+            context.logger.info("Received ALB request to path: \(albRequest.path)")
+
+            let response = ALBTargetGroupResponse(
+                statusCode: .ok,
+                headers: ["Content-Type": "application/json"],
+                body: "{\"source\":\"ALB\",\"path\":\"\(albRequest.path)\"}"
+            )
+
+            let encoder = JSONEncoder()
+            let responseData = try encoder.encode(response)
+            try await responseWriter.write(ByteBuffer(bytes: responseData))
+            try await responseWriter.finish()
+            return
+        }
+
+        // Try to decode as APIGatewayV2Request
+        if let apiGwRequest = try? decoder.decode(APIGatewayV2Request.self, from: data) {
+            context.logger.info("Received API Gateway V2 request to path: \(apiGwRequest.rawPath)")
+
+            let response = APIGatewayV2Response(
+                statusCode: .ok,
+                headers: ["Content-Type": "application/json"],
+                body: "{\"source\":\"APIGatewayV2\",\"path\":\"\(apiGwRequest.rawPath)\"}"
+            )
+
+            let encoder = JSONEncoder()
+            let responseData = try encoder.encode(response)
+            try await responseWriter.write(ByteBuffer(bytes: responseData))
+            try await responseWriter.finish()
+            return
+        }
+
+        // Unknown event type
+        context.logger.error("Unable to decode event as ALB or API Gateway V2 request")
+        throw LambdaError.invalidEvent
+    }
+}
+
+enum LambdaError: Error {
+    case invalidEvent
+}
+
+let runtime = LambdaRuntime(handler: MultiSourceHandler())
+try await runtime.run()