OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_ADDRESS_AUTO_SET_ENABLE macro

@def OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_ADDRESS_AUTO_SET_ENABLE Set to 1 for DNS client to automatically set and update the server IPv6 address in the default config (when it is not explicitly set by user). This feature requires SRP client and its auto-start feature to be also enabled. SRP client will then monitor the Thread Network Data for DNS/SRP Service entries to select an SRP server. The selected SRP server address is also set as the DNS server address in the default config.