feat(transport): add typed HTTP request interface in _requests_base.py

[cureprotocols]

This PR continues the effort from #1508 by introducing full type annotations to google/auth/transport/_requests_base.py.

✔️ Highlights:

• Defines a typed Request interface with a clear abstract contract for HTTP transport implementations.
• Includes docstrings and usage example for clarity and onboarding.
• Ensures strict Mypy compatibility under --strict mode.

✅ Clean run: mypy --strict google/auth/transport/_requests_base.py

Happy to refine if needed!

[google-cla]

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[cureprotocols]

I signed it!