Expose the keep_alive_sec attribute in HASS Developer Tools - States (#381)

* Typing: Make BaseThermostat generic on the UnderlyingEntity type

* Typing: Change the type of IntervalCaller._interval_sec from int to float

This makes the IntervalCaller class more reusable.

* Keep-alive: Expose UnderlyingSwitch.keep_alive_sec as a HASS Dev Tools attribute

Also improve a keep-alive log message.

custom_components/versatile_thermostat/underlyings.py

@@ -188,7 +188,7 @@ class UnderlyingSwitch(UnderlyingEntity):
         thermostat: Any,
         switch_entity_id: str,
         initial_delay_sec: int,
-        keep_alive_sec: int,
+        keep_alive_sec: float,
     ) -> None:
         """Initialize the underlying switch"""
 
@@ -217,6 +217,11 @@ class UnderlyingSwitch(UnderlyingEntity):
         """Tells if the switch command should be inversed"""
         return self._thermostat.is_inversed
 
+    @property
+    def keep_alive_sec(self) -> float:
+        """Return the switch keep-alive interval in seconds."""
+        return self._keep_alive.interval_sec
+
     @overrides
     def startup(self):
         super().startup()