Ah - this is a pretty painful point: Istio requires Kubernetes CRD wrapper metadata (primarily the name and namespace fields), but those fields are not part of the API objects themselves nor are they represented in the protos. (This is changing with the new MCP API for configuring components - which Galley uses - does encode these fields as protobufs but that doesn't help for your use case.) Instead, you should use the types in istio.io/istio/pilot/pkg/config/kube/crd, which implement the K8s CRD interface.

The easiest way to work with the Istio objects in golang is to use Pilot's libraries, particularly the istio.io/istio/pilot/pkg/model and istio.io/istio/pilot/pkg/config/kube/crd packages, as well as the model.Config struct. You can either pass around the full model.Config (not great because spec has type proto.Message so you need type assertions to extract the data you care about), or pass around the inner object wrap it in a model.Config before you push it. You can use the model.ProtoSchema type to help with conversion to and from YAML and JSON. Pilot only defines ProtoSchema objects for the networking API, the type is public and you can create them for arbitrary types.

So, using your example code I might try something like:

import (
    v1alpha1 "istio.io/api/rbac/v1alpha1"
   "istio.io/istio/pilot/pkg/model"
)


func getDefaultServiceRole() *v1alpha1.ServiceRole {
    return &v1alpha1.ServiceRole{
        Rules: []*v1alpha1.AccessRule{
            {
                Ports: []int32{2},
            },
        },
    }
}

func toConfig(app *nais.Application, role *v1alpha1.ServiceRole) model.Config {
    return &model.Config{
        ConfigMeta: model.ConfigMeta{
            Name:      app.Name,
            Namespace: app.Namespace,
        },
        Spec: app,
    }
}

type Client model.ConfigStore
func (c Client) CreateRoleFor(app nais.Application, role *v1alpha1.ServiceRole) error {
    cfg := toConfig(app, role)
    _, err := c.Create(cfg)
    return err
}

As a more complete example, we built the Istio CloudMap operator in this style. Here's the core of it that pushes config to K8s with Pilot libraries. Here's the incantation to create an instance of model.ConfigStore to use to create objects. Finally, I want to call out explicitly as it's only implicit in the example: when you call Create on the model.ConfigStore, the ConfigStore relies on the metadata in the ProtoSchema objects used to create it. So be sure to initialize the store with ProtoSchema objects for all of the types you'll be working with.

You can achieve the same using just the K8s client libraries and the istio.io/istio/pilot/pkg/config/kube/crd package, but I have not done it firsthand and don't have examples handy.