Description of problem:

Currently, the mirror-plugins.sh script needs to rebuild the plugin catalog index image (after changing the plugin refs to point to the target mirror registry): https://github.com/redhat-developer/rhdh-operator/blob/47c39eda120832e6dff2bdd3a9baca5abc439403/.rhdh/scripts/mirror-plugins.sh#L562

Instead, we could mimic the OCP ImageDigestMirrorSet (IDMS) behavior by instructing users to create a registries.conf file and mount it as an extra file in their RHDH init container. This would allow for transparent mirroring of the catalog index image and plugins, thus eliminating the need to modify the catalog index.
Unlike IDMS which works only on OCP and works at the cluster nodes level (for PodSpec image pulls), this would work on any cluster since it would be handled from inside the RHDH init container.
Plus, users could be able to customize the registries.conf as needed depending on their target mirror registry.