Add a setting for allowing/disallowing duplicate generic package uploads

Problem to solve

When using the GitLab Package Registry to publish generic packages, a duplicate package name/version can be uploaded. This may be great in some cases but not if the package should be immutable.

Proposal

Add a setting that will allow you to define (at the group level) whether or not duplicate generic packages are allowed and allot for exceptions.

Testing

Add an end to end level test similar to the one done for Maven