ethernet: add iperf example to test real bandwidth

1. Add command for Ethernet based on console component.
2. Make cmd_system and iperf a component that can be referenced by other examples
3. Add "version" command to cmd_system.c
4. Clean up the README.md in all ethernet examples[TW#26525]

examples/system/console/main/cmd_wifi.c

@@ -18,6 +18,7 @@
 #include "esp_wifi.h"
 #include "tcpip_adapter.h"
 #include "esp_event_loop.h"
+#include "cmd_wifi.h"
 
 static EventGroupHandle_t wifi_event_group;
 const int CONNECTED_BIT = BIT0;