iam/apiv1: allow GetIamPolicy and GetIamPolicy on Service Account resources

[diogoteles08]

Is your feature request related to a problem? Please describe.

I need to update the IAM policy of a ServiceAccount resource and currently I couldn't find any way to do it using the google-cloud-go.

I could find documentation on how to do it using gcloud client libraries in Python and Java, but not for Go.

Describe the solution you'd like

The solution I'd like is basically allow using https://pkg.go.dev/cloud.google.com/go/[email protected]/apiv1#example-IamPolicyClient.GetIamPolicy with a Service Account as a resource.

This is an example on how I tried to do so:

package main

import (
	"context"
	"log"

	iam "cloud.google.com/go/iam/apiv1"
	"cloud.google.com/go/iam/apiv1/iampb"
)
func main() {
	ctx := context.Background()
	iamClient, err := iam.NewIamPolicyClient(ctx)
        if err != nil {
	    log.Fatalf("failed to create IAM client: %s", err)
	}
	defer iamClient.Close()
	req := &iampb.GetIamPolicyRequest{
	    Resource: "projects/{PROJECT_ID}/serviceAccounts/{EMAIL_ADDRESS}", // I also tried the full resource as suggested in https://cloud.google.com/iam/docs/full-resource-names
            Options:  &iampb.GetPolicyOptions{RequestedPolicyVersion: 3}
	}

	res, err := c.GetIamPolicy(ctx, req)
	if err != nil {
            log.Fatalf("failed to get IAM policy: %s", err)
	}

	log.Printf("res: %v", res)
}

But I repeatedly get this error::

rpc error: code = Unimplemented desc = unexpected HTTP status code received from server: 404 (Not Found); transport: received unexpected content-type "text/html; charset=UTF-8"
exit status 1

I could observe this behavior on the version v1.1.13, that I usually work with, but also in latest (v1.3.0)

Describe alternatives you've considered

Weirdly enough, I could make it work using the library google.golang.org/api, but it's considered complete and it's only on maintenance mode. I followed the exemple here: https://cloud.google.com/iam/docs/reference/rest/v1/projects.serviceAccounts/getIamPolicy#go

Additional context

Similar issue: #8671