Remove Codable conformance for request or response-only types

[andrewheard]

• Conform to Encodable for request-only types
• Conform to Decodable for response-only types

In some cases, you may not need Codable's support for bidirectional encoding and decoding. For example, some apps only need to make calls to a remote network API and do not need to decode a response containing the same type. Declare conformance to Encodable if you only need to support the encoding of data. Conversely, declare conformance to Decodable if you only need to read data of a given type.

-- https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types#2904056

Context: It's easier to add Codable conformance to our public API contract later, if needed, than to remove it if not needed.