Context

Service and ServiceImport both have an AppProtocol field.

Specs:

• Service: https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.27/#serviceport-v1-core
• ServiceImport: https://github.com/kubernetes-sigs/mcs-api/blob/6c631f4730d095bdec8f9100f7a7d3732d0abe85/pkg/apis/v1alpha1/serviceimport.go#L91-L106

EG currently parses the AppProtocol field for Service resources.

Problem

During some refactoring in #5451, we found that EG does NOT parse AppProtocol from ServiceImport CR. https://github.com/envoyproxy/gateway/pull/5451/files#r1990414195

Ask

Can we apply the same AppProtocol parsing to ServiceImport that we currently support on Service?

I assume we should, as Service and ServiceImport are mostly interchangeable. But should we support the same AppProtocol values for both resources?

• Value kubernetes.io/h2c should definitely be supported, as it is explicitly listed in the spec for ServiceImport.AppProtocol.
• Though EG supports AppProtocol = "grpc" for Service. But where did this special value come from? It is not part of RFC-6335 or in https://kubernetes.io/docs/concepts/services-networking/service/#application-protocol. Is it OK to add support for this special value in ServiceImport.AppProtocol?