fix(types): add full type annotations and docstrings to transport init module

[cureprotocols]

📝 Description:

This PR contributes to issue #1508 by updating the google/auth/transport/__init__.py module to support full compliance with Mypy's --strict mode.

✔️ Highlights:

• ✅ Added complete type annotations to all functions.
• ✅ Included structured docstrings for clarity and usability.
• ✅ Verified clean with:

  mypy --strict google/auth/transport/__init__.py

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[cureprotocols]

I signed it!