server: refactor: introduce `AuthUser` interface

[paul-marechal]

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Replace the references to OAuth2User by AuthUser. This allows downstream extenders to more easily contribute alternative OAuth2 providers: If the expected data is stored in different attributes it will be possible to bridge it by implementing the proper AuthUser.

[paul-marechal]

Note that there seems to be an issue with formatting in this project...

I'm using VS Code along its Java extension to develop and it often feels like reorganizing or cleaning up imports.

I'd be in favor of keeping the cleanup as part of these changes.

It would be good to look into making auto-formatting part of the build step, maybe as a gradle plugin of some kind?

[paul-marechal]

Rebased my branch.

[amvanbaren]

@HeikoBoettger Can you share which additional identity provider you were able to implement using this PR?