json: add a new "sensitive" flags for JsonVariant objects

An object marked with this flag will be erased from memory when it is
freed. This is useful for dealing with sensitive data (key material,
passphrases) encoded in JSON objects.

src/shared/json.h

@@ -130,6 +130,8 @@ JsonVariant *json_variant_by_key_full(JsonVariant *v, const char *key, JsonVaria
 
 bool json_variant_equal(JsonVariant *a, JsonVariant *b);
 
+void json_variant_sensitive(JsonVariant *v);
+
 struct json_variant_foreach_state {
         JsonVariant *variant;
         size_t idx;