Add Geo file transfer validation flipper

So the feature can be quickly disabled in case of bug.

ee/lib/gitlab/geo/transfer.rb

@@ -147,6 +147,8 @@ module Gitlab
         # types of artifacts are not checksummed at all at the moment.
         return false if expected_checksum.blank?
 
+        return false unless Feature.enabled?(:geo_file_transfer_validation, default_enabled: true)
+
         expected_checksum != actual_checksum(file_path)
       end