Refactor APIGatewayXXX to eliminate duplicate code

[sebsto]

There is an opportunity to DRY by refactoring part of the APIGateway, APIGatewayV2, and APIGateway+WebSocket payload.

As per my assistant:

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1. HTTP Headers and Body Handling

• Properties like headers, multiValueHeaders (or just headers in V2), and body, as well as isBase64Encoded, appear in all major APIGateway request/response structs:
  • APIGatewayRequest
  • APIGatewayV2Request
  • APIGatewayWebSocketRequest
  • Their corresponding response types.

2. Context/Identity Structures

• Each request type defines a nested Context struct (sometimes with further nesting like Identity or HTTP), which contains information about the caller, API, stage, request IDs, etc.
• Many fields in these context structs overlap, such as apiId, stage, requestId, domainName, and sourceIp.

3. Codable/Sendable Conformance

• All these types conform to Codable (and, where supported, Sendable).

4. CodingKeys Mapping

• Several structs use custom CodingKeys enums to map from JSON input to struct properties, especially for context/requestContext fields.

5. Response Struct Pattern

• The response structs (APIGatewayResponse, APIGatewayV2Response, and by alias, APIGatewayWebSocketResponse) all follow a similar pattern: statusCode, headers, body, and isBase64Encoded, with slight variations.

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Candidates for Code Sharing/Abstraction

• Protocols: Define a protocol for common properties (headers, body, isBase64Encoded) and have each struct conform to it.

  protocol APIGatewayRequestCommon: Codable {
      var headers: HTTPHeaders? { get }
      var body: String? { get }
      var isBase64Encoded: Bool? { get }
  }

• Shared Context/Identity Structs: Refactor common context/identity fields into base structs or protocols, and compose them in each specific context struct as needed.
• Initializers/Decoding Helpers: Shared extensions or helper initializers to reduce repeated decoding logic for commonly structured properties.

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Summary Table of Common Properties:

Property	APIGatewayRequest	APIGatewayV2Request	APIGatewayWebSocketRequest
headers	✓	✓	✓
multiValueHeaders	✓	(V2 drops this)	✓
body	✓	✓	✓
isBase64Encoded	✓	✓	✓
context/requestContext	✓	✓	✓ (as context)
stage	✓ (in context)	✓ (in context)	✓ (in context)
apiId	✓ (in context)	✓ (in context)	✓ (in context)
requestId	✓ (in context)	✓ (in context)	✓ (in context)
domainName	✓ (in context)	✓ (in context)	✓ (in context)
sourceIp	✓ (in context)	✓ (in context)	✓ (in context.identity)