Pass k8s Service data through to the TCP balancer script.

Fixes broken L4 ExternalName services.

Details
---------

The `tcp_udp_balancer.lua` script checks if the property
`backend.service.spec["type"]` equals "ExternalName".  If so,
the script does a DNS lookup on the name in order to configure
the backend configuration.

However, before this commit, the k8s `Service` data was
_not_ set on the `Backend` struct passed into the `tcp_udp_balancer.lua`
script and therefore the ExternalName check always returned false.

This commit fixes the issue by setting the `Service` field on
the `Backend` struct. This also requires adding a new field to the
`L4Backend` struct first, so that it's available to set on the `Backend`.

internal/ingress/controller/controller.go

@@ -343,6 +343,7 @@ func (n *NGINXController) getStreamServices(configmapName string, proto apiv1.Pr
 				ProxyProtocol: svcProxyProtocol,
 			},
 			Endpoints: endps,
+			Service:   svc,
 		})
 	}
 	// Keep upstream order sorted to reduce unnecessary nginx config reloads.