Flatten and include external ids in contact custom fields

This is mainly needed for the Spoke/Nationbuilder integration, so may
as well keep it general for all external ids.

Author: michael-gratton

app/serializers/identity_spoke/spoke_member_sync_push_serializer.rb

@@ -35,6 +35,9 @@ def custom_fields
           representative_name: area.representative_name
         }
       }
+      @object.member_external_ids.each{ |external_id|
+        data[external_id.system + "_id"] = external_id.external_id
+      }
       data.to_json
     end