enhancement: support DNS query for different domains at different datacenter

my use case is :
dc1: app1.dc1.consul.local
dc2: app2.dc2.xyz.com

I want app2@dc2 can resolve app1.dc1.consul.local, and app1@dc1 can resolve app2.dc2.xyz.com.

Seems like dns query is only bound with each DC's domain name as below codes.
- https://github.com/hashicorp/consul/blob/master/command/agent/dns.go

- 95 // Register mux handlers

- 96 mux.HandleFunc(domain, srv.handleQuery)

It may be useful to support different domain name query cross multiple datacenter.