OIDC auth: Allow the source field of `username` to be configurable instead of fixed to `sub`

[NexZhu]

Feature Description

Currently in OIDC auth, Gitea get username from sub field returned from OIDC provider, which is often UUID. This should be configurable, for example to use preferred_username instead of sub.

Screenshots

[NexZhu]

Will this feature request be considered?

[helmut72]

I use a mapper in Keycloak to send content of preferred_username in sub field.

Gitea also doesn't support name field. Keycloak send full name in this field to Gitea, but it's empty in Gitea.