[ethernet] Restructure for multi-platform support

Restructure the ethernet component from ESP32-only to multi-platform,
following the same pattern as the wifi component (FILTER_SOURCE_FILES
with platform-specific .cpp files).

- Split ethernet_component.cpp into common code + ethernet_component_esp32.cpp
- Remove DEPENDENCIES = ["esp32"], add platform validators per type
- Add FILTER_SOURCE_FILES to select platform-specific .cpp
- Move ESP32 imports inside platform-conditional functions
- Update ethernet_info guards from USE_ESP32 to USE_ETHERNET
- Add USE_ETHERNET_SPI/OPENETH/SPI_POLLING_SUPPORT to defines.h
- Guard ethernet_helpers.c with USE_ESP32

No behavioral changes for ESP32 — this is a pure restructuring to
enable adding non-ESP32 platform support.

esphome/components/ethernet/ethernet_component_esp32.cpp

@@ -1,6 +1,7 @@
 #include "ethernet_component.h"
 
-#if defined(USE_ETHERNET) && defined(USE_ESP32)
+#ifdef USE_ETHERNET
+#ifdef USE_ESP32
 
 #include "esphome/core/application.h"
 #include "esphome/core/helpers.h"
@@ -838,4 +839,5 @@ void EthernetComponent::write_phy_register_(esp_eth_mac_t *mac, PHYRegister regi
 
 }  // namespace esphome::ethernet
 
-#endif  // USE_ETHERNET && USE_ESP32
+#endif  // USE_ESP32
+#endif  // USE_ETHERNET