Add structured-merge-diff vendor

This is where most of the logic actually lives.

vendor/sigs.k8s.io/structured-merge-diff/fieldpath/doc.go

@@ -0,0 +1,21 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package fieldpath defines a way for referencing path elements (e.g., an
+// index in an array, or a key in a map). It provides types for arranging these
+// into paths for referencing nested fields, and for grouping those into sets,
+// for referencing multiple nested fields.
+package fieldpath