Support /oauth/userinfo API (OpenID Connect)

[Amejonah1200]

One main problem with the /api/v4/user endpoint is that the permission needed is read_user which also gives permissions to read all users (/api/v4/users) also. This is kinda bad if the account doing the OAuth2 is an administrator account, because it is much more powerful.

Said differently: Support OIDC instead of "pure" OAuth2