Make the SDK Async-friendly

[dabeeeenster]

If folk want to use this SDK in an async/non-blocking environment like FastAPI they will not have a good time. We should move to using async-friendly libraries.

[tushar5526]

I like how sync and async clients are created in encode/httpx https://github.com/encode/httpx/blob/master/httpx/_client.py. Something similar can be used here, the only thing concerning would be a lot of duplicated codes, doc strings for Sync and async clients, but I don't think there are any workarounds for that.