Store tools as a set of strings (#365)

As noted the get_stored_tools(), StoredState cannot store arbitrary
objects; this commit applies the same logic to _on_redetect_hardware()

Fixes: #364

src/charm.py

@@ -115,7 +115,7 @@ def _on_redetect_hardware(self, event: ops.ActionEvent) -> None:
 
         if event.params["apply"] and hw_change_detected:
             # Update the value in local Store
-            self._stored.stored_tools = available_tools
+            self._stored.stored_tools = {tool.value for tool in available_tools}
             event.log(f"Run install hook with enable tools: {sorted_available_tools}")
             self._on_install_or_upgrade(event=event)