Unify member management between projects and groups - Owner Role

We treat projects and groups differently in multiple aspects:

• access levels:
  • for groups, we allow: from GUEST to OWNER
  • for projects, we allow: from GUEST only to MAINTAINER, but no OWNER

Because of this, we cannot grant other users OWNER privileges to a project. See #21432 (closed).

• owner:
  • for groups, we add owner to members with OWNER access level
  • for projects:
    • projects in personal namespaces: we don't add the owner as member but they are directly associated via Project#owner
    • projects in groups: owner is added inheriting access level from group (so they might end up with DEVELOPER access level only)

Because of this, we don't return the owner for personal projects via the members API. It's only return be the projects API via the owner attribute. There's a long discussion around this in https://gitlab.com/gitlab-com/gl-security/security-operations/sirt/operations/-/issues/657#note_399169972.

I assume most of these are coming from historic reasons but they are causing a lot of inconsistency, confusion and edge cases we need to handle. This might also be required for simplifying groups and project.