Add a diff kustomization feature

If implemented it will permit queriying the Kubernetes API to fetch the specified
Flux Kustomization, then uses the specified path to build the overlay.
It will then ssa-dry-run apply and output the diff using homeport/dyff

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/diff_kustomization.go

@@ -0,0 +1,77 @@
+/*
+Copyright 2021 The Flux 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 main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	"github.com/fluxcd/flux2/internal/kustomization"
+	kustomizev1 "github.com/fluxcd/kustomize-controller/api/v1beta2"
+)
+
+var diffKsCmd = &cobra.Command{
+	Use:     "kustomization",
+	Aliases: []string{"ks"},
+	Short:   "Diff Kustomization",
+	Long:    `The diff command does a build, then it performs a server-side dry-run and output the diff.`,
+	Example: `# Create a new overlay.
+flux diff kustomization my-app --path ./path/to/local/manifests`,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
+	RunE:              diffKsCmdRun,
+}
+
+type diffKsFlags struct {
+	path string
+}
+
+var diffKsArgs diffKsFlags
+
+func init() {
+	diffKsCmd.Flags().StringVar(&diffKsArgs.path, "path", "", "Name of a file containing a file to add to the kustomization file.)")
+	diffCmd.AddCommand(diffKsCmd)
+}
+
+func diffKsCmdRun(cmd *cobra.Command, args []string) error {
+	if len(args) < 1 {
+		return fmt.Errorf("%s name is required", kustomizationType.humanKind)
+	}
+	name := args[0]
+
+	if diffKsArgs.path == "" {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.path)
+	}
+
+	if fs, err := os.Stat(diffKsArgs.path); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid resource path %q", diffKsArgs.path)
+	}
+
+	builder, err := kustomization.NewBuilder(rootArgs.kubeconfig, rootArgs.kubecontext, rootArgs.namespace, name, diffKsArgs.path, kustomization.WithTimeout(rootArgs.timeout))
+	if err != nil {
+		return err
+	}
+
+	err = builder.Diff()
+	if err != nil {
+		return err
+	}
+
+	return nil
+
+}