gitserver.Client: Add a method to clone a repo from another gitserver instance

Created by: indradhanush

🚫 Blocked by #26490 (closed)

Add a new method that uses gitserer.protocol.RequestRepoUpdate under the hood and does the following for a given repo:

• Identify the current gitserver instance where the repo is located based on the existing hashing scheme
• Set protocol.RequestRepoUpdate.MigrateFrom to the value derived above
• Send a request to the new owner based on the rendezvous hashing scheme to clone the repo via gitserver.server.Server.handleRepoUpdate