Mask dockerconfigjson secret types and support StringData secrets

If implemented, flux diff kustomization will managed correctly sops
managed dockerconfigjson secrets.
Sops encrypted secret with stringData maps are supported too.

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

cmd/flux/testdata/diff-kustomization/dockerconfigjson-sops-secret.yaml

@@ -0,0 +1,11 @@
+apiVersion: v1
+data:
+  .dockerconfigjson: eyJtYXNrIjoiKipTT1BTKioifQ==
+kind: Secret
+metadata:
+  labels:
+    kustomize.toolkit.fluxcd.io/name: podinfo
+    kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
+  name: docker-secret
+  namespace: default
+type: kubernetes.io/dockerconfigjson