Add an encodable version of the APIGatewayResponse (#86)

Add a convenience initializer to the APIGatewayResponse v2 that accepts
an Encodable object

### Motivation:

Most Lambda developers will return a JSON object when exposing their
function through the API Gateway. The current initializer only accepts a
`String` body as per the AWS message definition.

This obliges all developers to write two lines of code + error handling
to encode their object and transform the `Data` to a string.

### Modifications:

Add a new initializer that accepts a `body` as `Encodable`

```swift
public init<Input: Encodable> (
        statusCode: HTTPResponse.Status,
        headers: HTTPHeaders? = nil,
        body: Input,
        isBase64Encoded: Bool? = nil,
        cookies: [String]? = nil
) throws { ... }
```

### Result:

Developers can now pass a Swift struct to the APIGatewayResponse.

```swift
let businessResponse = BusinessResponse(message: "Hello World", code: 200)
return APIGatewayV2Response(statusCode: .ok, body: businessResponse)
```

Sources/AWSLambdaEvents/APIGateway+Encodable.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright (c) 2017-2022 Apple Inc. and the SwiftAWSLambdaRuntime project authors
+// 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 HTTPTypes
+
+#if canImport(FoundationEssentials)
+import class FoundationEssentials.JSONEncoder
+import struct FoundationEssentials.Data
+#else
+import class Foundation.JSONEncoder
+import struct Foundation.Data
+#endif
+
+extension Encodable {
+    fileprivate func string() throws -> String {
+        let encoded = try JSONEncoder().encode(self)
+        return String(decoding: encoded, as: UTF8.self)
+    }
+}
+
+extension APIGatewayResponse {
+
+    public init<Input: Encodable>(
+        statusCode: HTTPResponse.Status,
+        headers: HTTPHeaders? = nil,
+        multiValueHeaders: HTTPMultiValueHeaders? = nil,
+        encodableBody: Input
+    ) throws {
+        self.init(
+            statusCode: statusCode,
+            headers: headers,
+            multiValueHeaders: multiValueHeaders,
+            body: try encodableBody.string(),
+            isBase64Encoded: nil
+        )
+    }
+}
+
+extension APIGatewayV2Response {
+
+    public init<Input: Encodable>(
+        statusCode: HTTPResponse.Status,
+        headers: HTTPHeaders? = nil,
+        encodableBody: Input,
+        cookies: [String]? = nil
+    ) throws {
+        self.init(
+            statusCode: statusCode,
+            headers: headers,
+            body: try encodableBody.string(),
+            isBase64Encoded: nil,
+            cookies: cookies
+        )
+    }
+}

Tests/AWSLambdaEventsTests/APIGateway+EncodableTests.swift

@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright (c) 2017-2020 Apple Inc. and the SwiftAWSLambdaRuntime project authors
+// 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 Foundation
+import Testing
+
+@testable import AWSLambdaEvents
+
+struct APIGatewayEncodableResponseTests {
+
+    // MARK: Encoding
+    struct BusinessResponse: Codable, Equatable {
+        let message: String
+        let code: Int
+    }
+
+    @Test
+    func testResponseEncodingV2() throws {
+
+        // given
+        let businessResponse = BusinessResponse(message: "Hello World", code: 200)
+
+        var response: APIGatewayV2Response? = nil
+        #expect(throws: Never.self) {
+            try response = APIGatewayV2Response(statusCode: .ok, encodableBody: businessResponse)
+        }
+        try #require(response?.body != nil)
+
+        // when
+        let body = response?.body?.data(using: .utf8)
+        try #require(body != nil)
+
+        #expect(throws: Never.self) {
+            let encodedBody = try JSONDecoder().decode(BusinessResponse.self, from: body!)
+
+            // then
+            #expect(encodedBody == businessResponse)
+        }
+    }
+
+    @Test
+    func testResponseEncoding() throws {
+
+        // given
+        let businessResponse = BusinessResponse(message: "Hello World", code: 200)
+
+        var response: APIGatewayResponse? = nil
+        #expect(throws: Never.self) {
+            try response = APIGatewayResponse(statusCode: .ok, encodableBody: businessResponse)
+        }
+        try #require(response?.body != nil)
+
+        // when
+        let body = response?.body?.data(using: .utf8)
+        try #require(body != nil)
+
+        #expect(throws: Never.self) {
+            let encodedBody = try JSONDecoder().decode(BusinessResponse.self, from: body!)
+
+            // then
+            #expect(encodedBody == businessResponse)
+        }
+    }
+}