Use user id from a trusted header in case mlflow runs behind proxy

[hahahannes]

Current State:
Authentication is done completely by the plugin.

New feature:
Having the possibility to use the user id from a header and to bypass the plugin authentication

My use case:
I am running MlFlow behind a proxy which already handles authentication. I can pass a header with the user email downstream to the plugin. It would be nice to use it. I know this plugin is mainly about OIDC authentication but I also really like the authorization feature.