Add AsyncParsableArguments to support validating async arguments

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.

Start with the existing ParsableArguments type and its validate() throws API to understand how shared argument validation currently works. The issue does not name files or tests and leaves the async API shape and completion criteria open, so those decisions need to be resolved before implementation.

Written by the indexing model from the issue text.