Add flux create secret proxy command

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/testdata/create_secret/proxy/secret-proxy.yaml

@@ -0,0 +1,11 @@
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: proxy-secret
+  namespace: my-namespace
+stringData:
+  address: https://my-proxy.com
+  password: my-password
+  username: my-username
+