update return type to Sequence

Author: ohmayr

packages/gapic-generator/gapic/schema/wrappers.py

@@ -2278,7 +2278,7 @@ def names(self) -> FrozenSet[str]:
         return frozenset(answer)
 
     @utils.cached_property
-    def resource_messages(self) -> FrozenSet[MessageType]:
+    def resource_messages(self) -> Sequence['MessageType']::
         """Returns all the resource message types used in all
         request and response fields in the service."""