Grant Admin Rights to custom SAML group

Is your feature request related to a problem? Please describe

When using SAML SSO, users must be added to the 'admin' group on the IdP as any changes made locally to the Nextcloud instance will be overwritten when the users logs in. There is no way to change or map a group to the 'admin' group in Nextcloud within the instance. This means the backend must have a group called 'admin' which does not clearly indicate what the group is for as it is likely the IdP is shared between multiple applications.

Describe the behaviour you'd like

There should be in option in the SAML configuration to grant members of a Group admin rights to the Nextcloud application. This way the IdP could have a group such as nextcloud-admins be associated with members who should have admin rights to Nextcloud.

Describe alternatives you've considered

Currently you can create a group within your IdP called 'admin'. For example, if using Keycloak you can create a local group called 'admin' and map the user to that group (you can do this with Roles as well).

Additional context

This community topic also describes the issue
https://help.nextcloud.com/t/how-to-give-saml-users-admin-rights/115468

Start by reviewing the SAML configuration and the existing handling of IdP groups, using the reported admin-group behavior as the baseline. Define how a configurable group such as “nextcloud-admins” should grant Nextcloud admin rights and verify that the mapping is not overwritten on subsequent logins.