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/stringdata-sops-secret.yaml

@@ -0,0 +1,12 @@
+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