connect: enable proxy.passthrough configuration

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 possible to "punch a hole" through the network namespace
to a specific HTTP or gRPC path, with the anticipated use case of creating
Consul checks on Connect enabled services.

A future PR may introduce more automagic behavior, where we can do things like

1) auto-fill the 'expose.path.local_path_port' with the default value of the
   'service.port' value for task-group level connect-enabled services.

2) automatically generate a port-mapping

3) enable an 'expose.checks' flag which automatically creates exposed endpoints
   for every compatible consul service check (http/grpc checks on connect
   enabled services).

api/services.go

@@ -168,8 +168,9 @@ type SidecarTask struct {
 
 // ConsulProxy represents a Consul Connect sidecar proxy jobspec stanza.
 type ConsulProxy struct {
-	LocalServiceAddress string `mapstructure:"local_service_address"`
-	LocalServicePort    int    `mapstructure:"local_service_port"`
+	LocalServiceAddress string              `mapstructure:"local_service_address"`
+	LocalServicePort    int                 `mapstructure:"local_service_port"`
+	ExposeConfig        *ConsulExposeConfig `mapstructure:"expose"`
 	Upstreams           []*ConsulUpstream
 	Config              map[string]interface{}
 }
@@ -179,3 +180,15 @@ type ConsulUpstream struct {
 	DestinationName string `mapstructure:"destination_name"`
 	LocalBindPort   int    `mapstructure:"local_bind_port"`
 }
+
+type ConsulExposeConfig struct {
+	Paths []*ConsulExposePath `mapstructure:"paths"`
+	// todo(shoenig): add magic for 'checks' option
+}
+
+type ConsulExposePath struct {
+	Path          string
+	Protocol      string
+	LocalPathPort int    `mapstructure:"local_path_port"`
+	ListenerPort  string `mapstructure:"listener_port"`
+}