Cells: Use Topology Service to identify the correct Cell for /allowed request

Overview

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

Topology Service is a gRPC server that provides an answer to the following question: To which cell an HTTP request related to a particular record (project, group, etc...) should be routed?

Requirement

This commit !969 (c4a952fc) partially does it:

• It adds Topology Service PoC as a local dependency
• Assumes that Topology Service is running locally
• Performs gRPC calls to the Topology Service and routes GitLab Shell HTTP calls based on the response

This issue is about making this functionality production-ready. But it's currently blocked by Topology Service production readiness.

Prepequisite

As demonstrated in !969 (c4a952fc) GitLab Shell is not flexible enough to customize the host of HTTP request. Feel free to create a separate issue/MR for refactoring.