What needs to get done

Global arguments should have a way to provide a limited set of choices or a type coercion for the application.

Two examples are the built-in --verbosity, which has a limited set of choices, and charmcraft's --project-dir, which provides a validator:

verbosity = GlobalArgument(
    "verbosity",
    "option",
    None,
    "--verbosity",
    "Set the verbosity level to 'quiet', 'brief', 'verbose', 'debug' or 'trace'",
    choices=[mode.name.lower() for mode in EmitterMode],
)
project_dir = GlobalArgument(
    "project_dir",
    "option",
    None,
    "--project_dir",
    "Set the base directory for the project.",
    validator=pydantic.DirectoryPath,
)

choices matches argparse, validator is chosen instead of type since that's already used.

Why it needs to get done

Right now an app needs to parse and provide error messages for this by itself, but craft-cli can often do this itself.