Add backends.GetProtocol func for mixed protocol

[TortillaZHawaii]

Similar to #2733, but this one only looks at the service definitions, for single protocol it won't change anything, so for example TCP will still have TCP backend, but when both TCP and UDP services are used it will use L3 backend service.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @TortillaZHawaii. Thanks for your PR.

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[TortillaZHawaii]

/assign @mmamczur

[mmamczur]

/ok-to-test