Enhancement: Implement real Pydantic v2 support

[SupImDos]

Summary

• As raised by @lucidfrontier45 in #48, we need Pydantic v2 support in pydantic-argparse.
• We now have pseudo-Pydantic v2 support, as the pydantic dependency is at least unpinned.
  • See: #55
  • See: v0.9.0
• However this is just a shim to the Pydantic v1 API.
• We should implement real Pydantic v2 support.
• I think maintaining both Pydantic v1 and v2 compatibility is probably unreasonable (TBD).
• More information to follow...

Notes

• Looks like our forks may have done some of the heavy-lifting
  • See: https://github.com/cody-mar10/pydantic-argparse
  • See: https://github.com/alejoe91/pydantic-argparse/tree/pydantic2