Bring back a custom dialer for Gitaly Ruby

Closes #1447 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53473

This change brings back the custom dialer for unix sockets, when communicating with Gitaly Ruby.

Unfortunately, the coverage around this part of the code is pretty low (as shown by the fact that this change passed the test stage of CI without any changes at all).

I'm considering what type of integration tests we should add to prove that this fix will stay fixed.