New RPC to check whether remote name exists

To fix https://gitlab.com/gitlab-org/gitlab-ce/issues/50562, we would like to check if the remote name exists on a repository before adding it unless we are okay with calling RemoteService#AddRemote and letting git complain with fatal: remote my-remote already exists.

I notice we have RemoteService#RemoteRepositoryExists, but this actually just checks whether the remote URL exists. I thought it would behave similarly to RemoteService#AddRemote or RemoteService#RemoteRemote, but it does not. I think we want to add RemoteService#RemoteExists here.