Is your feature request related to a problem? Please describe.
I have an external auth source and I'm using authentik as an authentication hub between the source and other applications. That auth source has unique user ids that I save in authentik as a custom attribute. I would like to use it as the oidc subject.

Describe the solution you'd like
Add a subject mode option "Based on a user attribute" with a text field where one enter the attribute. Alternatively it could be an expression similar to property mappings.

This would be quite similar to the current "Based on the User's UPN" and it may even make sense to replace it entirely, but that would require migrating existing configurations to the new type with upn as the attribute.

Describe alternatives you've considered
I could set the external uid as the username in authentik as I'm not currently using the username for anything