feat(transport): add typed HTTP request interface in _requests_base.py (#1508)

[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 a usage example to support onboarding and maintainability.
• Ensures strict compatibility with mypy --strict.

✅ Verified clean:

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

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