[gRPC/http2 auto-detect] Support auto-detection (or upgrade) in Activator

[rafaeltello]

Describe the feature

This is part of #4283. As part of supporting http2 auto-detect, we need to make changes in health probes, queue proxy, and activator. Below are some notes created by @igorbelianski on the work that needs to be done specifically in activator.

Data Path Notes

Activator always listens on both 8012 an 8013 constructs 2 distinct servers:

  [cmd/activator/main.go](https://github.com/knative/serving/tree/master/cmd/activator/main.go)

     servers := map[string]*http.Server{
	"http1":   pkgnet.NewServer(":"+strconv.Itoa(networking.BackendHTTPPort), ah),
	"h2c":     pkgnet.NewServer(":"+strconv.Itoa(networking.BackendHTTP2Port), ah),
          "profile": profiling.NewServer(profilingHandler),
	}

Queue proxy container construction

While construction queue proxy container we make a static choice on the port 8012/8013 pkg/reconciler/revision/resources/queue.go

func makeQueueContainer(rev *v1.Revision, cfg *config.Config) (*corev1.Container, error) {

[evankanderson]

/area networking /triage acecpted /kind enhancement

[knative-prow-robot]

@evankanderson: The label(s) triage/acecpted cannot be applied, because the repository doesn't have them.

In response to this:

/area networking /triage acecpted /kind enhancement

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[evankanderson]

/triage accepted

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