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/README.md

@@ -39,6 +39,12 @@ The above command will update the [OpenAPI schema] and the [Kustomization schema
 create a directory kubernetesapi/v1212 and store the resulting
 swagger.json and swagger.go files there.
 
+#### Precomputations
+
+To avoid expensive schema lookups, some functions have precomputed results based on the schema. Unit tests
+ensure these are kept in sync with the schema; if these tests fail you will need to follow the suggested diff
+to update the precomputed results.
+
 ### Run all tests
 
 At the top of the repository, run the tests.