Add ssh-upload-pack endpoint to handle Git over SSH requests

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/456116

**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.

Changelog: added
EE: true