Support async operations

[multimeric]

Currently the SDK is entirely synchronous, which is not desirable in async environments such as many web servers.

[sirosen]

Yep! This is on our long term roadmap, but we haven't had the kind of time it requires.
I've had some discussions with other members of the team about ideas for how to handle this. Many parts of the SDK need to be touched. In order to get proper type annotations, I think we ultimately will need some form of codegen.

In the meantime, I recommend wrapping any SDK usage in an async application with asyncio.to_thread. Because that propagates the async context, it should be possible to pass an SDK client to the thread if it's stored in a contextvar. (NB: I haven't actually tried that part.)