Make the HTTP library used for REST and GraphQL taps pluggable

While Requests has become the dominant HTTP(S) library for Python users, others are rising in popularity including httpx which offers a variety of improvements on Requests including support for HTTP/2. Requests is probably a good default due to how familiar it is for most Python developers, but it might be quite useful if there was a way to make the library pluggable.

There may be some potential code changes that might have to be made for HTTPX support—in particular, it doesn't support prepared requests. HTTPX also supports both sync and async (which may be useful if #184 (closed) is resolved and the concerns around debugging of async code are resolved).