Topology Service Client ruby gem is integrated in GitLab Rails

The purpose of this issue is to allow using Topology Service in GitLab Rails:

• Topology Service Client is scaffolded
• Topology Service once released published ruby gem with a client interface
• The Topology Service Client is integrated into GitLab Rails bundle. Done here !159778 (merged)
• Document the steps to keep the Topology Service Gem up to date in the Monolith Application. See the comment here for context.
• [ ] The GitLab Rails has easily accessible instance of all exposed Topology Service services. Moved into its own issue: #474660 (closed)