Support GitLab as a generic OIDC provider

[MaximilianHausen]

Description

Adds a workaround for gitlab as an oicd provider. Gitlab returns a non-standard created_at field during authentication that needs to be discarded before passing it on to the database schema, just like what already happens for keycloak. Also adds a gitlab icon for the provider id "gitlab".

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks (It should be fine, but I skipped the pre-commit checks because they weirdly capitalized an unrelated multi-line comment later in the file)
• Any AI code was thoroughly reviewed by me

[krokosik]

Why is Keycloak logic being modified? I don't have a Keycloak instance so I have no way of testing if it breaks.

[MaximilianHausen]

I changed it because it triggered a no-useless-computed-key linter warning, but didn't verify it further.

[MaximilianHausen]

I’ve now tested the keycloak sign-in with a local instance, and it still works.

[krokosik]

cool, thanks