fix(types): add type annotations to exceptions module for Mypy strict mode (#1508)

[cureprotocols]

This PR contributes to issue #1508 by adding full type annotations to google/auth/exceptions.py.

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

✔️ Highlights:

• Added explicit return types for all functions and properties.
• Replaced keyword-only syntax with **kwargs for Python 3.7 compatibility while preserving clarity and behavior.
• Maintained full backward compatibility with existing exception usage patterns.
• Verified clean via: mypy --strict google/auth/exceptions.py.

This is part of the ongoing effort to enhance strict typing across the library.
Happy to refine or expand this PR further as needed!

[google-cla]

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[cureprotocols]

I signed it!