GitLab CI, master branch (2021.11.24.)

Once again... wrote a GitLab CI configuration for the repository.

This was triggered by @fwinkl's request in !897 (merged) for adding Projects/TestProject to the CI. Which is something that currently would need to be done in Jenkins.

While sticky runners would make our lives a whole lot nicer for this repository as well, we can actually survive without them here. With the public runners the build will take very long. (I don't think the AnalysisBaseExternals build will finish inside of the 2 hour limit.) I still wanted to demonstrate what we could do right now with GitLab CI in this repository.

Especially note the commented setup for build:AnalysisBaseExternals:aarch64-centos7-gcc9-opt. I noticed that there's a lonely GitLab ARM64 runner with the label docker-arm. 😄 Which even sort of works. What it doesn't do is to allow access to CVMFS. 😦 So it can't really be used for our build at the moment. (For AnalysisBaseExternals it could be useful even without CVMFS, but for AthSimulationExternals it wouldn't be.)

This is really just me thinking out loud here... But we may still think about this a bit.

Pinging @aundrus, @emoyse, @elmsheus.