Add an endpoint to retrieve PATs for a given service account, specifically for top-level group owners

Add APIs to support service accounts and token ... (&16509 - closed) contains APIs used to support the new service account UI. We currently allow admins to fetch PATs for a specific user via API, which can be used by the UI on SM. On GitLab.com however, we need a new endpoint that will allow top-level group owners to fetch PATs for a particular service account.

This task is to add such an endpoint (see https://gitlab.com/gitlab-org/gitlab/-/blob/e8262fc0d3bc35f27dd973c52b6af234fc444031/ee/lib/api/group_service_accounts.rb#L158 for existing endpoints)