Support GRPC and HTTP2 without explicit port labeling

[rgregg]

In what area(s)?

/area networking

Describe the feature

Today to use GRPC or HTTP/2 for ingress into your service, you have to explicitly label the ports (see the GRPC sample: https://github.com/knative/docs/blob/master/docs/serving/samples/grpc-ping-go/sample.yaml)

However, the Runtime Contract states that the platform will perform automatic detection between HTTP/1.1, QUIC, and HTTP/2.

Unless overridden by setting the name field on the inbound port, the platform will perform automatic detection as described above. Developers SHOULD prefer to use automatic content negotiation where available