[Thread] Add NAT64 switch to Thread demo app

This CL adds a switch to the Thread demo app that allows the user to enable or disable NAT64.

See screenshot https://screenshot.googleplex.com/3UDJ6wXqRf6WKRy.

Bug: 375559106
Change-Id: I2a5c9f08ef664e62a5c34987eedb321e2b01fd85
2 files changed