Create a FindTag RPC

When the Rails application wants a single tag, it requests all of them and selects the one by name it needs.

This is inefficient and something we should filter them on the Gitaly side. The RPC could be:

rpc FindTag(FindTagRequest) returns (FindTagResponse)

message FindTagRequest {
  Repository repository = 1;
  bytes tag_name = 2;
}

message FindTagResponse {
  Tag tag = 1;
}