SaaS: API to search for group members by `extern_uid`

Proposal

Submitting on behalf of a GitLab SaaS user:

For Gitlab SaaS, as a group owner, we need an API to be able to search group members by extern_uid:

Use case: We need our automation to be able to search for group members based on SAML extern_uid so they can be removed from our Gitlab group immediately when the employee is terminated.

Impact: Since Gitlab sessions can last several hours, we need an API call to identify and remove an employee, based on their SAML Extern_ID, automatically from our Gitlab group as soon as the employee is terminated. We don’t use SCIM.