Add new netd event callback for changes to NAT64 prefix

Bug: 78545619
Test: system/netd/tests/runtests.sh pass
Test: Register a callback in ConnectivityService temporarily and print out
event parameters. Check event parameters as expected in logs while NAT64 prefix
is added and removed.

Change-Id: I11b2b0aeb159d5fe8dc01ccf6dc64bd9d4d960a5