connect: enable setting connect upstream destination namespace

[shoenig]

This PR enables setting destination_namespace for a Connect proxy upstream.

[shoenig]

Quick example

➜ consul version 
Consul v1.12.1+ent
Revision cfd128ff
Protocol 2 spoken by default, understands 2 to 3 (agent will automatically use protocol >2 when speaking to compatible agents)

Using this PR cherry-picked onto nomad-enterpise because I'm too lazy to register Consul services by hand

➜ nomad version 
Nomad v1.3.2-dev+ent (10a88fa6cae490a24dac7403f381c1e8b34ae280+CHANGES)

consul namespace create -name ns1
consul namespace create -name ns2

job file

job "countdash" {
  datacenters = ["dc1"]

  group "api" {
    network {
      mode = "bridge"
    }

    consul {
      namespace = "ns1"
    }

    service {
      name = "count-api"
      port = "9001"

      connect {
        sidecar_service {}
      }
    }

    task "web" {
      driver = "docker"

      config {
        image = "hashicorpnomad/counter-api:v3"
      }
    }
  }

  group "dashboard" {
    network {
      mode = "bridge"

      port "http" {
        static = 9002
        to     = 9002
      }
    }

    consul {
      namespace = "ns2"
    }

    service {
      name = "count-dashboard"
      port = "9002"

      connect {consul namespace create -name ns1
consul namespace create -name ns2

        sidecar_service {
          proxy {
            upstreams {
	      destination_name = "count-api"
	      destination_namespace = "ns1"
              local_bind_port  = 8080
            }
          }
        }
      }
    }

    task "dashboard" {
      driver = "docker"

      env {
        COUNTING_SERVICE_URL = "http://${NOMAD_UPSTREAM_ADDR_count_api}"
      }

      config {
        image = "hashicorpnomad/counter-dashboard:v3"
      }
    }
  }
}

[tgross]

LGTM!

[github-actions]

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