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/build-kustomization/podinfo-result.yaml

@@ -123,6 +123,31 @@ spec:
   type: ClusterIP
 ---
 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
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  labels:
+    kustomize.toolkit.fluxcd.io/name: podinfo
+    kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
+  name: secret-basic-auth-stringdata
+  namespace: default
+stringData:
+  password: KipTT1BTKio=
+  username: KipTT1BTKio=
+type: kubernetes.io/basic-auth
+---
+apiVersion: v1
 data:
   token: KipTT1BTKio=
 kind: Secret