Apply sign field for cherry pick

In previous issues (#570019 (closed)) a sign field was added to UserCherrypick.

The current behaviour is to sign the commit when the gitaly_gpg_signing flag is enabled or the sign field is true. The UserCherrypick rpcs needs to be updated to use the sign field to determine whether or not the resulting commit will be signed. The commit should not be signed whengitaly_gpg_signing is enabled but he sign field is false.