New method for downloading streams

My current thought is to change the interface signature and make Download work differently. Right now, it returns Task<Stream?>, which prevents the user to inspect the response for errors. It crashes if the call wasn't successful.

So, the idea is to make ExecuteStreamAsync, which will return Task<RestStreamResponse>. The response will be like RestResponse but instead of Content it will have Stream?. Naturally, it requires to be IDisposable.

The first step there was to create a source generator to create instance of RestResponseBase child class, as copying those properties manually is error prone (adding a new prop requires many changes). That part is done, so now this thing should be relatively easy.

Originally posted by @alexeyzimarev in https://github.com/restsharp/RestSharp/issues/2072#issuecomment-2174572685

Start by tracing the existing Download method and the proposed ExecuteStreamAsync entry point, then review RestResponse, RestResponseBase, and RestStreamResponse, including the completed source generator. Done means stream downloads expose response errors and a disposable response with Stream? rather than returning only Task<Stream?>.