argx should preserve signature of wrapped function

[aiven-amartin]

In https://github.com/aiven/aiven-client/blob/46f2cfd7c674fdfb62e277ede33fd77283f9292a/aiven/client/argx.py#L88, we don't cast back to the signature of the wrapped function, but we should, as well as appending the type of the added args.

See https://mypy.readthedocs.io/en/stable/generics.html#declaring-decorators.

[NotoriousPyro]

def wrap(func) should also ideally have @functools.wraps decorator.

[rominf]

#322 is the fix.