Roborock: Add clean_percent

Note: Only newer models have this (Q Revo is the only one I know of).  HA shows the sensor as `unknown` for other models; I don't know how to exclude it.

miio/integrations/roborock/vacuum/vacuumcontainers.py

@@ -204,6 +204,14 @@ def vacuum_state(self) -> VacuumState:
         """Return vacuum state."""
         return STATE_CODE_TO_VACUUMSTATE.get(self.state_code, VacuumState.Unknown)
 
+    @property
+    @sensor("Cleaning Progress", icon="mdi:progress-check", unit="%")
+    def clean_percent(self) -> Optional[int]:
+        """Return progress of the current clean."""
+        if "clean_percent" in self.data:
+            return int(self.data["clean_percent"])
+        return None
+
     @property
     @sensor(
         "Error code",