New method for downloading streams

[alexeyzimarev]

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 #2072 (comment)