Add dynamic completion suppport

This commit adds dynamic completion support for the following commands
and flags:

- `flux delete ...` command
- `flux export ...` command
- `flux get ...` command
- `flux reconcile ...` command
- `flux resume ...` command
- `flux suspend ...` command
- `--namespace` flag
- `--context` flag

Signed-off-by: Kazuki Suda <kazuki.suda@gmail.com>

cmd/flux/export_kustomization.go

@@ -33,6 +33,7 @@ var exportKsCmd = &cobra.Command{
 
   # Export a Kustomization
   flux export kustomization my-app > kustomization.yaml`,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
 	RunE: exportCommand{
 		object: kustomizationAdapter{&kustomizev1.Kustomization{}},
 		list:   kustomizationListAdapter{&kustomizev1.KustomizationList{}},