Add async programming to Boa Constrictor

[AutomationPanda]

Right now, Boa Constrictor's code is entirely synchronous. It would be nice to have async support. Async will be required if we want to add Playwright support (#71).

I think the best design would be to create interfaces for AsyncQuestion and AsyncTask. IActor would need async AsksFor and AttemptsTo methods to call these async interactions. I'm open to suggestions.

Microsoft C# async programming guide:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/