Privileged issue

• I'm @tiangolo or he asked me directly to create an issue here.

Issue Content

I want Typer to have optional support for async functions.

It would depend on having AnyIO (or maybe Asyncer 🤔) installed. If the command function is async, then it would run it with anyio.run().

Maybe autocompletion functions could also be async, so they would have to be checked to see if they need to be called directly or awaited.