Override Git HTTP user agent

What does this MR do?

For analytics it's nice to understand who is interacting with Git repositories. For example in the case of the GitLab Runner, it uses the same interface any other user has as it uses the normal Git. This makes the metrics gathering harder. For example, for GitLab.com we currently use Google IP ranges as heuristic.

This change overrides the git user agent, so for SRE's and Gitaly developers it's easier to understand what clones are from GitLab-CI, versus the other clones.

What's the best way to test this MR?

One way is through running a job in this branch and checking if the new user agent will show up in Kibana. This would only work for GitLab Inc employees.

What are the relevant issue numbers?

Closes: #26754 (closed)