GitLab Rails: Replace Geo Git over HTTP calls with new Workhorse HTTP endpoint for SSH

Overview

The general information can be found in Cells: SSH Routing for Git Pull and Push (&13532)

This issue is about introducing the GitLab Rails changes necessary to make this issue possible: Geo: Replace Git over HTTP calls with new Workh... (gitlab-shell#762 - closed). It requires adding a feature flag and sending additional data from GitLab Rails.

Draft: Poc: HTTP endpoint for fetching gRPC git... (!146227 - closed) sends git_rpc_auth_header and git_rpc_url fields. We can use the fields to send Geo AND Cells data.

Note

Cells changes may be blocked by Global Service production readiness, so let's focus on general and Geo side of things for now.