[FEATURE] Support OAuth User Attribute Mapping

[lightlike]

Is your feature request related to a problem? Please describe.

I want to connect tinyauth to nextcloud but it seems that the user endpoint does not return all the data (email in this case) in the expected structure.

Describe the solution you'd like.

It would be nice to have some simple json mapping where one can just remap the location of the provided userinfo.

Example:

TINYAUTH_OAUTH_PROVIDERS_[NAME]_USERINFOPATH_EMAIL=ocs.data.email

Additional context

There already is a similar Issue with Grafana: https://help.nextcloud.com/t/oauth2-userinfo-api/70556/12

An example structure for nextcloud user can be found here: https://docs.nextcloud.com/server/latest/developer_manual/client_apis/OCS/ocs-api-overview.html#user-metadata

The docs show xml but it can be formatted as json using format=json

Human Written Confirmation

• I confirm this request was written by me and not generated by an LLM or AI assistant.