Add ability to pass in Custom GitLab secrets

Follow up from #645 (closed), #653 (closed) and #794 (closed).

In 3.0.0 we were able to remove the dependency on NFS in GET. This required the switch to a new method to propagate GitLab Secrets.

The new method was designed in the hope that it would allow for easier direct support for users to passthrough custom secrets as desired, which the previous method was not congruent with.

!1140 (merged) laid the groundwork in part on how to handle this for situations when a Geo primary was not controlled by GET but it's secrets had to be passed to the GET controlled secondary.

Task is to explore if this ability can be expanded to all environment types.