fix(client): Add type ignore for async stub casting in Assets API client

- Added a type ignore comment in the `stub` property to handle the casting from a sync stub to an async stub.
- This change ensures compatibility with the async implementation while maintaining type safety.

These modifications improve the functionality of the Assets API client by addressing type issues rel

Author: eduardiazf

src/frequenz/client/assets/_client.py

@@ -57,7 +57,9 @@ def stub(self) -> assets_pb2_grpc.PlatformAssetsAsyncStub:
         """
         if self._channel is None or self._stub is None:
             raise ClientNotConnected(server_url=self.server_url, operation="stub")
-        return self._stub
+        # This type: ignore is needed because the stub is a sync stub, but we need to
+        # use the async stub, so we cast the sync stub to the async stub.
+        return self._stub  # type: ignore
 
     async def get_microgrid_details(  # noqa: DOC502 (raises ApiClientError indirectly)
         self, microgrid_id: int