The PR sync the main and async branch.

---------

Co-authored-by: Chandra Shekhar Sirimala <chandrasiri@google.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: Chalmer Lowe <chalmerlowe@google.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Adds async credential wrapper for async client. 

Context:

- The standard google auth credentials are currently synchronous, and
it's asynchronous credentials classes are either not available or marked
private.
- Credential retrieval and refreshing will remain synchronous under the
hood. Rationale: As authentication tokens typically possess an
expiration lifetime of one hour, the blocking time required for token
fetching occurs infrequently. The performance impact of blocking (or
utilizing a separate thread for offloading) once per hour is deemed
negligible when weighed against the considerable engineering cost of
developing and maintaining a asynchronous authentication.

This PR introduces an abstract class to support the upcoming async
client for the Python SDK. This refactor defines the public interface
and enables code sharing between sync and async client, without
introducing new logic.

Add AsyncConnection class and unit tests