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

[cureprotocols]

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

✔️ Highlights:

• Annotated all exception classes with explicit return types.
• Replaced keyword-only arguments with **kwargs for Python 3.7 compatibility.
• Ensured backward compatibility and preserved existing exception behavior.
• Verified clean using: mypy --strict google/auth/exceptions.py

This continues the ongoing effort to fully type this library and improve static analysis support.

Let me know if any refinements are needed!