ot::Dns::Client::ResetDefaultConfig() method

Resets the default config to the config used when the OpenThread stack starts. When OpenThread stack starts, the default DNS query config is determined from a set of OT config options such as `OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_IP6_ADDRESS`, `_DEFAULT_SERVER_PORT`, or `_DEFAULT_RESPONSE_TIMEOUT` etc. (see `config/dns_client.h` for all related config options).