Add support for verification of the X-Gitlab-Token HTTP header.
This pull request will prepare the module for data-in-modules and provide a REFERENCE.md. Unfortunately this module uses a lot of classes and I need to figure out which one are private. Because some variables are named or used badly, this change will be backwards incompatible and end up in a major release.
This MR includes the following:
The current documentation for triggering the webhook via curl only specifies an example for non-prefixed environments. With the current data structure format, it's unclear whether the full environment name needs to be specified in the 'ref' parameter. This PR implements clear documentation, noting that the full environment name is required.