esp_modem: Add option to support for LCP keepalive

Closes https://github.com/espressif/esp-idf/issues/6562

examples/protocols/pppos_client/main/Kconfig.projbuild

@@ -54,6 +54,14 @@ menu "Example Configuration"
                 Enter the peer phone number that you want to send message to.
     endif
 
+    config EXAMPLE_LCP_ECHO
+        bool "Enable LCP ECHO"
+        default n
+        select LWIP_ENABLE_LCP_ECHO
+        help
+            Enable this option to make use of LCP keepalive using
+            LCP_ECHO_INTERVAL and LCP_MAXECHOFAILS to default values
+
     menu "UART Configuration"
         config EXAMPLE_MODEM_UART_TX_PIN
             int "TXD Pin Number"