Custom source type

[waclaw66]

Hi!
I would like to implement login via my country local OAuth2 provider (seznam.cz). Unfortunatelly I'm unable to configure that, becuse it uses special scope called "identity" without common identification field like "id" or "sub", it has own field "oauth_user_id".
Is there a way how to create own source type like patreon or override user identification field?
Thanks in advance.