feat: add httpx transport; add compute_engine async

[bkanuka]

Opening a draft PR to gauge interest in adding an httpx transport.

This is used internally for a project that otherwise relied on httpx (instead of aiohttp) for async http requests. Using this httpx transport means that the response from an AuthorisedSession is an httpx.Respose like the other responses in our code.

Frankly, this a copy/paste job from _aiohttp_requests.py with relevant parts changed.

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compute_engine._credentials_async.py is also added for async Compute Engine credentials. Arguably, this should be discussed separately, but was created while developing the httpx transport.

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There is more work to be done before this could merge, but please discuss and provide feedback.

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[clundin25]

Super cool! I will give this a thorough read later.

Two high level questions I have for you.

1. What made you decide to use HTTPX instead of aiohttp?
2. Is it feasible to support both with the same code?
   • I will be digging into this later.

[Lawouach]

Oh this is exactly what I'd need. httpx is such a brilliant library, it would be awesome of the Google auth client could natively support it.

[chalmerlowe]

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:

• 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.

[chalmerlowe]

Added an issue to track this post migration.

See Issue #1908