gitserver: generate clone url from external service config

Created by: asdine

We currently build clone urls during external service synchronization and store them in the external_service_repos table. Since these urls might contain plaintext tokens, we want to stop storing them in that table anymore. This change focuses on making gitserver build clone urls on demand using the encrypted external service configuration. It introduces a new function types.RepoCloneURL in the types package to do so.