Allow choosing a custom role for gitlab_group_ldap_link

Apologies if this is supported and I'm missing something - looks like GitLab supports mapping an LDAP group to a custom role via UI and API (although API docs imply it's possible, it isn't explicitly listed/shown). However I don't believe the gitlab terraform gitlab_group_ldap_link resource supports this yet and it would be very useful!

"group_access (String) Minimum access level for members of the LDAP group. Valid values are: no one, minimal, guest, reporter, developer, maintainer, owner"