OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_NAT64_ALLOWED macro

@def OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_NAT64_ALLOWED Specifies the default NAT64 mode, i.e., whether to allow or disallow NAT64 address translation during DNS client address resolution. This mode is only available when `OPENTHREAD_CONFIG_DNS_CLIENT_NAT64_ENABLE` is enabled.