SSO / OIDC support

[samip5]

I'm reposting this to track progress.

I think it would be better for new implementations to implement MSC3861.

[NinekoTheCat]

This is just a whole can of worms of half-merged and merged branches? this issue probably should be split into individual spec impls

[jevolk]

There is a button to create sub-issues, I just don't know what to create. But it would be great to see this broken down into specifics if that helps.

[NinekoTheCat]

https://github.com/matrix-org/matrix-spec-proposals/pull/3861/files#diff-6d544bf0beaaf932d5a7642c06070c418e4b6ef29aba051aa00809f992677628R185-R189

I will create sub-issues for each of these

[jevolk]

Thank you so much 🤗