gitserver: Support MigrateFrom in RepoUpdateRequest

Created by: indradhanush

Issue created from discussion #26416.

We would like to clone a repo from one gitserver instance to another as part of the repo migration to the new hashing scheme. The first step towards this will be to add the ability to send a clone request to gitserver with the config to clone from another gitserver instance instead of the upstream repo URL of the external service. To do this, add:

• A MigrateFrom string attribute to gitserver.protocol.RepoUpdateRequest
• A MigrateFrom string attribute to gitserver.server.cloneOptions

And finally modify:

• gitserver.server.handleRepoUpdate to parse protocol.RepoUpdateRequest.MigrateFrom and set it to server.cloneOption.MigrateFrom so that in a subsequent PR we can make the decision to clone from a gitserver if this attribute is a non zero string.