…870)

* httpx async_send_each prototype

* Clean up code and lint

* fix: Add extra dependancy for http2

* fix: reset message batch limit to 500

* fix: Add new import to `setup.py`

…ntial backoff and `Retry-After` header (#871)

* Refactored retry config to `_retry.py` and added support for backoff and Retry-After

* Added unit tests for `_retry.py`

* Updated unit tests for HTTPX request errors

* Address review comments

Reverted previous timing adjustments for 'ExpiredTokenShort' and
'ExpiredCookieShort' and their corresponding test assertion skews
in tests/test_token_gen.py.

Implemented mocking of `google.auth._helpers.utcnow` in:
- TestVerifyIdToken.test_expired_token_with_tolerance
- TestVerifySessionCookie.test_expired_cookie_with_tolerance

This approach provides precise control over the 'current time' (`now`)
used during token verification logic, making the tests deterministic
and resilient to variations in execution speed (e.g., in PyPy).

The tests now accurately check token expiration logic against
defined clock skews without being affected by real-world timing
of test execution. Original token expiration deltas (T-30s) and
test skews are restored and correctly evaluated with mocked time.