Add allow-credentials to preflight header

[abidlabs]

I've tested and this fixes the issue that @hannahblair I identified. I don't think this introduces any security risk based on my mental model (as long as the origins are trusted, we do indeed want to pass on cookies and authorization headers).

[gradio-pr-bot]

🪼 branch checks and previews

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	Website	ready!	Website preview
:unicorn:	Changes	detected!	Details

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Install Gradio from this PR

pip install https://gradio-builds.s3.amazonaws.com/f17b2fdbe0b9281c9c712a9b90aaa7146cbc270f/gradio-4.31.5-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@f17b2fdbe0b9281c9c712a9b90aaa7146cbc270f#subdirectory=client/python"

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	patch

• [ ] Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

Add allow-credentials to preflight header

Maintainers or the PR author can modify the PR title to modify this entry.

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- Maintainers can change the version label to modify the version bump. - If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can [update the changelog file directly](https://github.com/gradio-app/gradio/edit/add-allow-credentials-header/.changeset/dark-streets-visit.md).

[abidlabs]

Thanks @hannahblair @pngwn!