CustomAction support
Description
A new CustomAction class that allows custom logic to be executed by gitlab-shell without needing the logic to exist within gitlab-shell.
This MR is necessary to support proxying of SSH git push requests from a Geo secondary to a Geo primary.
Builds upon !210 (merged) - Refactor for re-usability for future MR's
What are the relevant issue numbers?
Edited by Ash McKenzie