Precompute IsNamespaceScoped to avoid expensive schema reads (#4152)

* Precompute IsNamespaceScoped to avoid expensive schema reads

See https://github.com/GoogleContainerTools/kpt/issues/2469

For the `gcr.io/kpt-fn/set-namespace:v0.1` function, over 50% of CPU
time is spent on IsNamespaceScoped. Instead of unmarshalling 100k lines
of JSON to determine this, instead just precompute it. We can ensure
this never is inaccurate as the test verifies the precomputed result is
up to date.

In real world kpt pipelines this cuts execution of set-namespace (and
similar functions, just an example of a trivial function) from 2.0s to
1.0s. Because these functions are run in long pipelines over many
resources, this adds up a lot.

* Add documentation

kyaml/openapi/openapi_test.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/stretchr/testify/assert"
 	"sigs.k8s.io/kustomize/kyaml/yaml"
 )
@@ -271,6 +272,14 @@ func TestIsNamespaceScoped_builtin(t *testing.T) {
 	}
 }
 
+// TestIsNamespaceScopedPrecompute checks that the precomputed result meets the actual result
+func TestIsNamespaceScopedPrecompute(t *testing.T) {
+	initSchema()
+	if diff := cmp.Diff(globalSchema.namespaceabilityByResourceType, precomputedIsNamespaceScoped); diff != "" {
+		t.Fatalf(diff)
+	}
+}
+
 func TestIsNamespaceScoped_custom(t *testing.T) {
 	SuppressBuiltInSchemaUse()
 	err := AddSchema([]byte(`