feat(backend): support oidc token exchange

[michaeljguarino]

Summary

This allows for validation of external oidc id tokens and exchange w/ a plural token. We'll be able to add support for this in the cli and build a proper temporary token system w/in gh actions off it.

Test Plan

unit tests

Checklist

• [ ] If required, I have updated the Plural documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] I have added a meaningful title and summary to convey the impact of this PR to a user.
• [ ] I have added relevant labels to this PR to help with categorization for release notes.

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