remove duplication from component_iterator (#7210)

Co-authored-by: Samuel Tardieu <sam@rfc1149.net>
Co-authored-by: J. Nick Koston <nick@koston.org>

esphome/core/component_iterator.h

@@ -171,6 +171,11 @@ class ComponentIterator {
   } state_{IteratorState::NONE};
   uint16_t at_{0};  // Supports up to 65,535 entities per type
   bool include_internal_{false};
+
+  template<typename PlatformItem>
+  void process_platform_item_(const std::vector<PlatformItem *> &items,
+                              bool (ComponentIterator::*on_item)(PlatformItem *));
+  void advance_platform_();
 };
 
 }  // namespace esphome