This PR implements first-class support for Nomad running Consul
Connect Mesh Gateways. Mesh gateways enable services in the Connect
mesh to make cross-DC connections via gateways, where each datacenter
may not have full node interconnectivity.

Consul docs with more information:
https://www.consul.io/docs/connect/gateways/mesh-gateway

The following group level service block can be used to establish
a Connect mesh gateway.

service {
  connect {
    gateway {
      mesh {
        // no configuration
      }
    }
  }
}

Services can make use of a mesh gateway by configuring so in their
upstream blocks, e.g.

service {
  connect {
    sidecar_service {
      proxy {
        upstreams {
          destination_name = "<service>"
          local_bind_port  = <port>
          datacenter       = "<datacenter>"
          mesh_gateway {
            mode = "<mode>"
          }
        }
      }
    }
  }
}

Typical use of a mesh gateway is to create a bridge bet...