Implement OpenID Connect on top of current OAuth2.0 implementations

[Lgmrszd]

Is your suggested enhancement related to a problem? Please describe.

I wanted to add Modrinth as auth provider to my git (forgejo) instance, but right now, it seems that most applications don't support adding oauth2 sources with arbitrary endpoints.

Describe the solution you'd like

It seems that those applications that support adding custom auth OAuth2.0 sources (so far I've checked Forgejo and Authentik) do that by using OpenID-Connect built on top of it. So, implementing that would allow easy integration of Modrinth auth into 3rd party websites.

Describe alternatives you've considered

• Applications just adding Modrinth OAuth2.0 support instead (see additional context), but that is not universal
• Providing endpoints compatible with other services. For example, authentik provides Github-compatible endpoints for OAuth2.0 clients supporting Github Enterprise but not OpenID-Connect

Additional context

My initial requests to forgejo and upstream auth library:
forgejo Modrinth auth source request: https://codeberg.org/forgejo/forgejo/issues/3734
goth library Modrinth auth source request: markbates/goth#555