Add async support

[iamkubi]

This would only be feasible if it leaves the existing behavior intact as synchronous calls. There are ways to write the code as async and have a wrapper that generates a synchronous version. This could potentially be refactored so that pyadctyl exposes get_servers() and async_get_servers() without impacting the existing behavior.

Some relevant links

https://discuss.python.org/t/how-can-async-support-dispatch-between-sync-and-async-variants-of-the-same-code/15014/14
https://gist.github.com/blink1073/969aeba85f32c285235750626f2eadd8