Use `with_annotated` decorator with custom types for allow easy type conversion

[neoniobium]

Feature Request

When using the argparse approach (with_argparse decorator), the type argument of add_argument can be used for non-basic type conversion (see for example). The same is not possible when using the with_annotated decorator.

Possible design for a solution

Either allow custom types (as Typer does — see custom types) and make a best-effort type conversion for arbitrary annotations, or add an additional parameter to Option and Argument (for example conversion) that enables a two-step type conversion approach.

[KelvinChung2000]

I will likely add a "converter" option. I am considering forbidding the use of the type argument within the metadata, since this kind of adds a chance of shooting yourself by accident, like doing a: Annotated[str, Argument(type=int)] which does not make sense.

[neoniobium]

Forbidding the use of the type argument within the metadata seems reasonable to me — I actually wondered about it while testing the decorator.