Add a setting for allowing/disallowing duplicate Helm charts

Context

As part of the epic, &5070, this issue proposes adding a setting to control how the helm chart registry handles duplicates.

Problem to solve

In #337446 (closed), one of the problems raised is that duplicate uploads are not rejected. But this is a design decision. However, GitLab should offer a setting to allow Admin to customize this behavior.

Proposal

Add a setting to allow/prevent helm chart uploads.