[Self-hosted] Allow linking an existing user account to an OIDC identity

currently, when logging in via OIDC, there's no option to link (neither from admin or the users side) a current account with a OIDC identity. logging in via OIDC creates a new account.

either be able to manually link accounts from an admin's side, or be able to auto provision (of course, very insecure). in my case i would be happy with just being able to manually link from admin side, since i only have a small community for friends, although i am not sure how to implement it.

perhaps one could take inspiration from tuwunel's (matrix server implementation ) OIDC implementation for existing accounts?