[Refactor] Internalize konfig constants

This PR is an effort towards internalizing public APIs.
It moves some of the builtinconstants to internal/

Signed-off-by: Varsha Prasad Narsing <varshaprasad96@gmail.com>

api/internal/konfig/builtinpluginconsts/namespace.go

@@ -0,0 +1,20 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package builtinpluginconsts
+
+const (
+	namespaceFieldSpecs = `
+namespace:
+- path: metadata/name
+  kind: Namespace
+  create: true
+- path: spec/service/namespace
+  group: apiregistration.k8s.io
+  kind: APIService
+  create: true
+- path: spec/conversion/webhook/clientConfig/service/namespace
+  group: apiextensions.k8s.io
+  kind: CustomResourceDefinition
+`
+)