Add mDNS responder so we can utilize go-discovers mdns discovery for auto-joining

The go-discover mdns provider already supports discovering services using multicast DNS. This can be useful in testing networks or in other on-prem networks where multicast traffic can be used to automatically join to THE cluster.

This issue is to track adding an mDNS responder to Consul to allow returning the Consul instances advertise address when queried. The responder + using the go-discover provider with retry-join would allow auto-clustering of all instances on a local network without having to know any of the IPs ahead of time.