Add On/Off and Away mode support to template platform

Implemented support for On/Off and Away modes in the template water heater
platform, including optimistic control and lambda-based state reporting.

Refactored the base 'WaterHeaterCall' to replace the 'state_' bitmask with
'optional<bool>' for 'on' and 'away' fields. This change was necessary to
enable partial (delta) updates. The previous bitmask implementation did not
distinguish between a field being "set to false" and "not set at all,"
causing unintended state resets (e.g., turning the device off when only
adjusting temperature).

tests/components/template/common-base.yaml

@@ -414,6 +414,8 @@ water_heater:
     current_temperature: !lambda "return 42.0f;"
     target_temperature: !lambda "return 60.0f;"
     mode: !lambda "return water_heater::WATER_HEATER_MODE_ECO;"
+    on: !lambda "return true;"
+    away: !lambda "return false;"
     supported_modes:
       - "OFF"
       - ECO
@@ -424,6 +426,14 @@ water_heater:
       - PERFORMANCE
     set_action:
       - logger.log: "set_action"
+      - water_heater.template.publish:
+          id: template_water_heater
+          on: false
+          away: true
+      - water_heater.template.publish:
+          id: template_water_heater
+          on: !lambda "return true;"
+          away: !lambda "return false;"
 
 datetime:
   - platform: template