notifyBuildStatus: implement our own GitLab notification using Groovy
Turns out, addGitLabMRComment() doesn't really work in our situation, so we have to write our own code.
The code is implemented fully in Groovy, taking advantage that global pipeline library is not subjected to the sandbox. Mimic the code from GitLab branch source, and call into the GitLab4J and GitLab branch source plugin.
Edited by Ratchanan Srirattanamet