[water_heater] (1/4) Implement API/Core/component for new water_heater component (#12498)

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>
Co-authored-by: J. Nick Koston <nick+github@koston.org>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>
Co-authored-by: J. Nick Koston <nick@koston.org>

esphome/core/component_iterator.cpp

@@ -163,6 +163,12 @@ void ComponentIterator::advance() {
       break;
 #endif
 
+#ifdef USE_WATER_HEATER
+    case IteratorState::WATER_HEATER:
+      this->process_platform_item_(App.get_water_heaters(), &ComponentIterator::on_water_heater);
+      break;
+#endif
+
 #ifdef USE_EVENT
     case IteratorState::EVENT:
       this->process_platform_item_(App.get_events(), &ComponentIterator::on_event);