Add helpers to handle OAuth in a FastAPI app

[Wauplin]

This PR imports the code to handle OAuth in gradio to huggingface_hub. Goal is to make it available for non-Gradio apps (say autotrain-advanced for instance @abhishekkrthakur).

I also added tests to make sure the workflow still works correctly. Thanks @coyotte508 for huggingface/huggingface.js#1050 and advices :)

Missing parts:

• docstrings and documentation

I have flagged this feature as "experimental" to be able to introduce breaking changes in the future.
Note that for now, the integration only works for Spaces integrations but the same codebase could be tweaked to handle any kind of websites. That's lower priority though (the JS integration is much better for that).

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Demo: https://huggingface.co/spaces/Wauplin/fastapi-oauth

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.