ILB mixed protocol forwarding rules

[TortillaZHawaii]

When the mixed protocol flag is enabled this will use forwardingrules.EnsureILBIPv4.

In draft because this PR will likely be split into multiple smaller PRs and also waits for the backends.GetProtocol.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @TortillaZHawaii. Thanks for your PR.

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