Allow for a service to discover its Connect Proxies address via DNS

#### Feature Description

Right now Consul Connect makes assumptions about a proxy being deployed as a sidecar and where the application can just hard code a constant IP and port to reach some upstream service.

If the proxy and the application do not have static IPs but rather can "roam" then this assumption fails to hold and the application can lose connectivity to its proxied upstream.

We should consider adding the capability for an application to discover its proxy via DNS. I am not sure what the QNAME would look like yet. Maybe `..connect-internal..`. That still needs more thought.

A bonus I haven't given tons of thought to yet would be to synthesize SRV RRs including for each upstream of the proxy.

#### Use Case(s)

When you deploy a proxy in a different network namespace from the application. Even if you determine the IP of the proxy you want the application to use its still an extra manual step when we already have the proxy service registered. It would be a nice UX to be able to allow applications to use a DNS name and Consul's DNS