Fixes broken L4 ExternalName services.
Details
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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`.
f0173f0822