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/CMakeLists.txt

@@ -0,0 +1,6 @@
+# The following lines of boilerplate have to be in your project's CMakeLists
+# in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.16)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(slip_client)