Add AsyncParsableArguments to support validating async arguments

[luispadron]

Currently to share arguments across commands you can create a ParsableArguments type which contains your arguments/options, and can use func validate() throws to validate the arguments.

This fails for things that must be validated in an async manner. We should consider adding an async API for validating or AsyncParsableArguments to support this.

[CrazyFanFan]

I just encountered the same problem.

[dcantah]

Also just hit this.. thought I might've been holding it wrong. I can try and take a look