Support migration from Charts-based to Operator-based installation

Summary

Ideally, we'd support having the Operator take over an installation that was created via the GitLab Helm Charts. The current long-term goal is to eventually deprecate the existing GitLab Helm Charts and use the Operator as the primary Cloud Native installation method. When that happens, it would make the changeover much smoother for administrators if the Operator could simply take over an existing installation.

Acceptance criteria

• Charts-based installation can be overtaken by the GitLab Operator
• When the installation is taken over, all expected objects continue to exist and can be replaced by the Operator when needed