[BE] Adjust LDAP sync to sync admin custom roles

In #524879 (closed) we are adding model to store the mapping between LDAP groups and admin custom roles.

Now we need to sync the users from LDAP and assign them respective admin custom roles.

The work will likely be related to ee/lib/ee/gitlab/auth/ldap/access.rb , ee/app/workers/ldap_group_sync_worker.rb + new class for admin sync will need to be create 🤔

Additionally we should add a field to Users::UserMemberRole indicating the user is assigned to a custom role because of LDAP sync.