Support BufferRequests in HttpClient based backends

[tygill]

This change uses the HttpCompletionOptions to support the BufferRequests (--http-enable-buffering) setting in HttpClient based backends.

Both .Net and Mono appear to treat HttpCompletionOption.ResponseContentRead as a signal to cache the response, and HttpCompletionOption.ResponseHeadersRead as a signal to not cache the response.

Note that this will also change the default behavior to not buffer, as previously HttpClient based backends (which looks like it is just the MicrosoftGraphBackend's derivatives) would use the default, which was to have responses buffered.

This change is largely motivated by Duplicati throwing out of memory errors which seem to be related to attempting to allocate a memory stream to cache the full response when running on a Raspberry Pi with limited memory.

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/onedrive-v2-memory-leak/6112/29