[Cherry-pick #2575 -> 1.28] Forward individual ports for NetLB with 5 or less service ports

[TheBeeZee]

GCE forwarding rules have a limit of 5 forwarded ports. In the external L4 load balancer, we currently turn a set of exposed ports into a port range from minPort to maxPort and forward all ports in the range, which needlessly forwards traffic for ports that are not supposed to be exposed.

With this change, if 5 or less ports are exposed, we expose each distinct, otherwise we use the old mechanism of exposing a port range. This is similar to have the Internal Load Balancer is setup, except the ILB exposes all ports if more than 5 ports need to be exposed

[k8s-ci-robot]

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[sawsa307]

/ok-to-test

[k8s-ci-robot]

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