Refactor Auth(n/z) backend

[gjedlicska]

The current auth backend has a very low bus factor, we should rewrite it in TS to improve understanding in the team.

Also consider standardizing around OIDC. All major providers, Google, Github, AAD etc all support it, and our implementation would only focus on using the OIDC standard.
It would require a bit of rethinking how we configure a list of OIDC providers, but we could delete the special provider based implementations.