Add registry import subcommand to Omnibus' gitlab-ctl

Context

Similar to Add registry migration subcommand to Omnibus' g... (#1108 - closed)

As we move Allow manual implementation of container regist... (&10757) forward, we will need to expose a way for admins to execute the import steps using known tools for Omnibus installations.

Solution

Provide a subcommand to gitlab-ctl such as registry-database import that can execute the import steps as documented in https://gitlab.com/gitlab-org/container-registry/-/blob/master/docs/database-import-tool.md?ref_type=heads#importing-existing-data-to-the-database.