Workhorse: HTTP endpoint for bidirectional SSH git-upload-pack data

What does this MR do and why?

Contributes to #456116 (closed)

GitLab-Shell client: gitlab-shell!1081 (merged)

Problem

For Cells and Geo needs we want to route Git over SSH requests to the correct instance.

Solution

Introduce an HTTP Workhorse endpoint that accepts requests coming from GitLab-Shell and responds with git upload pack data.