Switch HTTPClient response handler from a closure to a protocol

[guoye-zhang]

Opening this to start a discussion. The new signature of perform:

    mutating func perform<ResponseHandler: HTTPClientResponseHandler & ~Copyable, Return: ~Copyable>(
        request: HTTPRequest,
        body: consuming HTTPClientRequestBody<RequestWriter>?,
        options: RequestOptions,
        responseHandler: consuming ResponseHandler
    ) async throws -> Return
    where
        ResponseHandler.ResponseConcludingReader: ~Copyable,
        ResponseHandler.ResponseConcludingReader == ResponseConcludingReader,
        ResponseHandler.Return == Return

Resolves #147

Remaining:

• Update docc comments
• Decide what to do with convenience methods
• Add support for informational responses in URLSession and AHC
• Update proposal

[guoye-zhang]

I was not able to get it to work with primary associated types due to SuppressedAssociatedTypesWithDefaults requiring ~Copyable for ResponseHandler.ResponseConcludingReader

[FranzBusch]

What do we expect a user to do in here? Is this just for them to observe it? Can they take any action on an informational response?

[guoye-zhang]

It depends on the status code. E.g. start sending the request body after 100 continue (the backing implementation can also handle this by delaying calling the closure until 100 continue); prefetching a resource after 103 early hints.