Improve the experience of using dataclasses and plain functions, add attrs support

[fleonce]

This PR makes the following changes:

• Adds support for attrs
• Removes custom parse function support (as this is one of attrs best features [attrs.Converter], etc.)
• Improve the typing experience, especially for strict functions that do not allow any additional arguments being passed at runtime
• Improve the CLI experience by passing the annotated type names as a metavar to argparse
• Rewrites the parsing logic to make argument passing + CLI parsing more robust
• Fixes some weird issues with kwonly arguments by streamlining the parsing process