Fixed #35303 -- Added async auth backends and associated functionality

[bigfootjon]

Trac ticket number

ticket-35303

Branch description

This is the culmination of the project to asyncify the contrib.auth module. This PR contains 3 things:

1. An update to the BaseBackend and all built-in backends (and related code) to provide async logic
2. An update to the documented (public) auth interface to use these new backends
3. An improvement to the RemoteUser middleware to take advantage of the new async-native logic

Each item above is a separate commit below, for easier convenience in reviewing. If requested, I'm happy to split this PR up into multiple PRs or combine commits.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• For UI changes, I have attached screenshots in both light and dark modes.

[nessita]

Hi Jon! Thank you for your contribution.

I see this is a draft PR, please remember that when this work is ready for review, you need to adjust the ticket flags in the Trac system so this PR gets added to the "branches needing review" section of the Django Developer Dahsboard. More information in these docs.

[carltongibson]

Hey @bigfootjon — Thanks for this. It looks good.

I don't think there's anything we can do about it — certainly not to block this PR about it! – but, I still find the duplication over the function color a bit… erm… sad. 🙂 We've got ≈identical skeletons differing in ≈only whether the internal helper is awaited or not. It would be nice to reduce that where possible over time.

About half-way down, that caused me to start scanning a little, so I can't say I looked at every code path, but looks good at the high level. 👍

[sarahboyce]

Hi @bigfootjon 👋 thank you for this PR
I have reviewed the list of new features with @nessita and there are some other PRs which we are already in progress reviewing and will likely struggle to review this in time for the 5.1 feature freeze on 22nd May.
Planning to pick this up after the feature freeze 👍