Google Cloud / Google Workspace oauth2 no preferred_username

### Problem Statement Google oauth2 no preferred_username in response - idtoken_preferred_username="" and user_preferred_username="" So common_name is using instead Related to https://github.com/jkroepke/openvpn-auth-oauth2/pull/437 and https://github.com/jkroepke/openvpn-auth-oauth2/blob/main/internal/oauth2/handler.go#L195 ### Proposed Solution Please make it configurable ### Additional information _No response_ ### Acceptance Criteria _No response_ ### Preflight Checklist - [x] I could not find a solution in the [documentation](https://github.com/jkroepke/openvpn-auth-oauth2/wiki), the [FAQ](https://github.com/jkroepke/openvpn-auth-oauth2/wiki/FAQ), the existing issues or discussions.