[routing-manager] generate random nat64 prefix and add it to netdata (#7232)

This commit generates a random NAT64 prefix and adds the prefix to
NetworkData if none exits. The prefix will be saved in Settings for
recovery.

It also adds a new CLI command `br nat64prefix` to show the local
nat64 prefix.

A new config OPENTHREAD_CONFIG_BORDER_ROUTING_NAT64_ENABLE is defined
and used to guard the change.

This initial implementation only supports a single BR.

script/check-scan-build

@@ -37,6 +37,7 @@ readonly OT_BUILD_OPTIONS=(
     "-DOT_BORDER_AGENT=ON"
     "-DOT_BORDER_ROUTER=ON"
     "-DOT_BORDER_ROUTING=ON"
+    "-DOT_BORDER_ROUTING_NAT64=ON"
     "-DOT_COAP=ON"
     "-DOT_COAP_BLOCK=ON"
     "-DOT_COAP_OBSERVE=ON"

script/make-pretty

@@ -83,6 +83,7 @@ readonly OT_CLANG_TIDY_BUILD_OPTS=(
     '-DOT_BORDER_AGENT=ON'
     '-DOT_BORDER_ROUTER=ON'
     '-DOT_BORDER_ROUTING=ON'
+    '-DOT_BORDER_ROUTING_NAT64=ON'
     '-DOT_CHANNEL_MANAGER=ON'
     '-DOT_CHANNEL_MONITOR=ON'
     '-DOT_CHILD_SUPERVISION=ON'