Use uint64 type for component IDs

The type used in gz-sim for component IDs is uint64_t which should match
the type used here.

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

Author: Addisu Z. Taddese

proto/gz/msgs/serialized_map.proto

@@ -32,7 +32,7 @@ message SerializedEntityMap
   uint64 id = 1;
 
   /// \brief All the components belonging to the entity.
-  map<int64, SerializedComponent> components = 2;
+  map<uint64, SerializedComponent> components = 2;
 
   /// \brief Whether the entity and all its components should be removed at the
   /// current state.