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/types.go

@@ -322,6 +322,8 @@ type L4Service struct {
 	Backend L4Backend `json:"backend"`
 	// Endpoints active endpoints of the service
 	Endpoints []Endpoint `json:"endpoints,omitempty"`
+	// k8s Service
+	Service *apiv1.Service `json:"service,omitempty"`
 }
 
 // L4Backend describes the kubernetes service behind L4 Ingress service