Generic Static Accessors in Swift 5.5

[jshier]

Goals ⚽

This PR makes various protocol values available as static members so they can be used in generic contexts using the new Swift 5.5 syntax. This enables some nicer call sites:

Previously

AF.request("https://httpbin.org/post",
            method: .post,
            parameters: Value(),
            encoder: JSONParameterEncoder(),
            interceptor: RetryPolicy())
    .response(responseSerializer: DecodableResponseSerializer<TestResponse>()) { response in
         debugPrint(response)
    }

Now

AF.request("https://httpbin.org/post",
           method: .post,
           parameters: Value(),
           encoder: .json,
           interceptor: .retryPolicy)
    .response(responseSerializer: .decodable(of: TestResponse.self)) { response in
        debugPrint(response)
    }

These improvements grow the more customized protocol values are used.

Testing Details 🔍

Added tests for accessing, but not using, these static values, as the types themselves are already tested.