feat: add httpx transport and async compute engine credentials

[google-labs-jules]

NOTE: closing this temporarily to support the migration to the python mono-repo. Once this repo is safely merged, this should be revisited.

Add httpx transport and async compute engine credentials.

This PR builds on the concepts, ideas, and code found in PR googleapis/google-auth-library-python#1255. There were some fundamental issues with the previous PR and we attempt to address those issues here. See this comment from the other PR:

This PR has been here a long time.

* I feel like there is relevance to this PR, support for `httpx` seems like a valid and
useful addition.

Having said that, we are actively pushing to migrate most of our libraries to our python
monorepo** (which means we need to disposition all Open PRs). I recommend we close this PR
for the moment and tackle this project as soon as the migration to the mono-repo is complete.

To move this concept to something that can be incorporated in the library, I strongly
recommend the action items listed below. I leveraged Jules to attempt to do these things and
Jules issued a PR that builds on this foundation and attempts to resolve the following. That
PR can be found here.

• Remove the coupling to httpx (_metadata_async.py imports explicitly from google.auth.transport._httpx_requests). This breaks the transport agnostic design of the library. If a user wanted to use aiohttp with async compute engine credentials, they would be unable to do so easily with this design.
• Ensure that _metadata_async.py implements a reasonable Request protocol that should accept any valid async request callable (such as aiohttp), not just httpx.
• Add unit tests.
• Remove all uses of datetime.datetime.utcnow() which has been deprecated (in Python 3.12) and replace them with google.auth._helpers.utcnow().

** One of the reasons for this push is to reduce the duplicative toil we have across all
our split repos. Reducing time on toil frees us up to focus more on adding features such as
this.

Supports closure of Issue googleapis/google-cloud-python#15134

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PR created automatically by Jules for task 5612836208709637744 started by @chalmerlowe

[google-labs-jules]

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