Enable configuration of HTTP and gRPC endpoints which should be exposed by
the Connect sidecar proxy. This changeset is the first "non-magical" pass
that lays the groundwork for enabling Consul service checks for tasks
running in a network namespace because they are Connect-enabled. The changes
here provide for full configuration of the

  connect {
    sidecar_service {
      proxy {
        expose {
          paths = [{
		path = <exposed endpoint>
                protocol = <http or grpc>
                local_path_port = <local endpoint port>
                listener_port = <inbound mesh port>
	  }, ... ]
       }
    }
  }

stanza. Everything from `expose` and below is new, and partially implements
the precedent set by Consul:
  https://www.consul.io/docs/connect/registration/service-registration.html#expose-paths-configuration-reference

Combined with a task-group level network port-mapping in the form:

  port "exposeExample" { to = -1 }

it is now pos...