Add "reference" transforms for object types

These are applied to all types using the
`convertToMultiNamespaceTypeVersion` field when defining any object
type. Any time the Kibana version is changed, all reference
transforms for applicable versions are applied, and all documents'
`referencesMigrationVersion` fields are updated to match the Kibana
version.

src/core/server/saved_objects/migrations/core/build_active_mappings.ts

@@ -164,6 +164,9 @@ function defaultMapping(): IndexMapping {
           },
         },
       },
+      referencesMigrationVersion: {
+        type: 'keyword',
+      },
     },
   };
 }