Add support for providing pipeline variables via git push options

Problem to solve

In omnibus-gitlab pipelines, the decision of whether a CE package is to be built or an EE one is taken by several factors. One override for that is the ee variable, upon the presence of which, an EE package is built. More than often, we need to build an EE package for testing, and the current workflow involves either polluting the .gitlab-ci.yml file with ee variable, or going to the UI and manually running a pipeline with ee variable set. If the commit doesn't have any CI skip directive, this means we are essentially running an additional unwanted pipeline.

Proposal

Make it possible to pass CI variables using git's push options. This way, while pushing the code to the repo we can specify the variables to be used in the pipeline, thus not polluting the CI config file, or make the trouble of going to the UI to start a pipeline.

Intended users

Developers