fix(types): add full type annotations to webauthn_types for Mypy stri…

[cureprotocols]

📝 Description

This PR contributes to issue #1508 by applying full type annotations to google/oauth2/webauthn_types.py.

All functions, methods, and class properties now comply with Mypy's --strict mode.

✅ Highlights:

• Annotated all method signatures and dataclass fields.
• Handled all Optional, Union, Dict, and List types accurately.
• Maintained full compatibility with Python 3.7+ and the existing library behavior.
• Verified clean with:

mypy --strict google/oauth2/webauthn_types.py

This is part of the ongoing effort to improve type safety and maintainability across the library. Let me know if anything needs refining!