Connect: Provide a dev proxy that can route to cluster via mesh-gateway

Mesh-gateway enables an easy way to have shared dev clusters that can be accessed from services running locally on dev laptops.

This is pretty speculative but has been requested by a few people so capturing the request here.

This would involve running a local proxy in a similar way to out built-in one allows for simple connection to consul services at L4.

A L7 dev proxy though could:
- use Envoy as implementation (for supported OSes, ideally via Docker as we have no binaries outside docker)
- a single proxy could listen for HTTP(2) traffic and use hostnames like `foo.service.consul.local` to route traffic to that service running in a shared cluster upstream via their mesh-gateway. In a sense the devs laptop would be like an instance in another DC but without having to run their own DC and federate it over WAN with correct encryption setup etc.

Start by examining Consul's existing built-in local L4 proxy and the mesh-gateway behavior described in the issue. Define how a Docker-based Envoy proxy would accept HTTP/2 hostnames such as foo.service.consul.local and route them to shared-cluster services; done means a supported local development path with clear OS and routing boundaries.

Written by the indexing model from the issue text.