Add CI variables for extracting identifying Git info

While there is GITLAB_USER_EMAIL, it is not very useful for identifying the author or the committer of the relevant code changes. This variable often contains useless information (i.e. the commits are made by a bot / associated with automation Gitlab account).

There are situations where Git-related information can be lost: multi-project pipelines don't allow one to dynamically create variables passed down to the next project, and the only information thus available is the email of Gitlab user who started the first job. As mentioned above, in many instances, the job is started by a bot / from an account not associated with the code changes it relates to. Git, however, stores both the author of a commit and the information about the person who pushed it.