Add Sign field to UserSquash RPC

Add a Sign field to the UserSquash RPC.

This field should control if the commit created by the rpc should be signed.. Since commit signing is hardcoded true currently, whether or the commit as a result of squashing is signed is controlled by the gitaly_gpg_signing flag. Updating this field to actually change whether the commit is signed will be handled in a separate issue after gitlab is already sending the field to gitaly.