Backport allow_mirror_update for Chain::Command

This backports the allow_mirror_update update field for Chain::Command
from EE to CE. There's no particular reason to keep this in EE. Doing so
would require refactoring of CE as there currently is no easy way to
inject new Struct members into the Chain::Command class.

lib/gitlab/ci/pipeline/chain/command.rb

@@ -11,7 +11,7 @@ module Gitlab
           :trigger_request, :schedule, :merge_request,
           :ignore_skip_ci, :save_incompleted,
           :seeds_block, :variables_attributes, :push_options,
-          :chat_data
+          :chat_data, :allow_mirror_update
         ) do
           include Gitlab::Utils::StrongMemoize