WIP: Add SSO via OIDC

[smndtrl]

What kind of change does this PR introduce?

SSO via OIDC in additional to SAML

What is the current behavior?

SSO only with SAML

What is the new behavior?

SSO with SAML or OIDC possible

Additional context

This is a WIP! Not all necessary functionality is in there.

The GenericProvider was taken from @carlobeltrame PR #1372

[hf]

Wow! Ideally we'd want this to be built by the team, but if you want to continue developing it we can have a sync some time?

[smndtrl]

@hf Feel free to take of the code what you want/need depending on your level of NIH. There is no real novel code to begin with as I made an effort to reuse as much of the existing code paths as possible.

I'm running my modified version on-top of a hosted supabase instance and that's all I needed for now. I did it mostly to re-familiarize myself with all the OIDC flows, state keeping, etc... For a lot of things I'm working on auth has made decisions that are too opinionated to be used further down the line.

[nietsmmar]

Wow! Ideally we'd want this to be built by the team, but if you want to continue developing it we can have a sync some time?

I heard (some months back). that this is actually on the Roadmap. Are there any more details on when this will be implemented? It is quite crucial for us to be able to use some specific OIDC providers. With supabase not supporting this, this is a big flaw for us. We would really appreciate to have this feature.

Thanks for all your work!