feat(examples): Move the slip example to esp-netif folder

And update the slip_modem_get_ipv6_address() to return the address by
copy instead of reference (to employ return value optimizaiton where
possible)

examples/esp_netif/slip_custom_netif/main/Kconfig.projbuild

@@ -0,0 +1,38 @@
+menu "Example Configuration"
+
+    menu "UART Configuration"
+        config EXAMPLE_UART_TX_PIN
+            int "TXD Pin Number"
+            default 25
+            range 0 36
+            help
+                Pin number of UART TX.
+
+        config EXAMPLE_UART_RX_PIN
+            int "RXD Pin Number"
+            default 26
+            range 0 36
+            help
+                Pin number of UART RX.
+
+        config EXAMPLE_UART_BAUD
+            int "UART baud rate"
+            default 115200
+            help
+                Baud rate for UART communication
+
+    endmenu
+
+    config EXAMPLE_UDP_PORT
+        int "Port for UDP echo server"
+        default 5678
+        help
+            Port for UDP echo server in example
+
+    config EXAMPLE_IPV4
+        bool "Test with IPv4 address"
+        default n
+        help
+            Test interface using IPv4
+
+endmenu