Support for on_off_gradually v2+ (#793)

Previously, only v1 of on_off_gradually is supported, and the newer versions are not backwards compatible.
This PR adds support for the newer versions of the component, and implements `number` type for `Feature` to expose the transition time selection.
This also adds a new `supported_version` property to the main module API.

kasa/smart/smartmodule.py

@@ -80,3 +80,8 @@ class SmartModule(Module):
             return next(iter(filtered_data.values()))
 
         return filtered_data
+
+    @property
+    def supported_version(self) -> int:
+        """Return version supported by the device."""
+        return self._device._components[self.REQUIRED_COMPONENT]