Add forwardingrules.GetILBProtocol func for mixed protocol

[TortillaZHawaii]

Used behind MixedProtocol feature flag for ILBs only. Requires #2743 to be merged as well.

[TortillaZHawaii]

/lint
/assign @mmamczur

[k8s-ci-robot]

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[k8s-ci-robot]

@TortillaZHawaii: 0 warnings.

In response to this:

/lint
/assign @mmamczur

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[mmamczur]

/ok-to-test

[mmamczur]

/ok-to-test

[mmamczur]

/lgtm

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