Support HTTP/2

[mdemirhan]

Expected Behavior

We have support only for HTTP/1.1 currently. Need to add an option to support HTTP/2 endpoints. Istio support for HTTP/2 is already there and we shouldn't have any blockers on our side to add this feature.

[evankanderson]

I think this is blocked on #1047

/kind feature

[mattmoor]

/remove-area API

Can you update this with whether/how the control plane needs to be aware of this? If it does, feel free to add back the API label.

[mattmoor]

/milestone Serving 0.4