MXS-1220: Factor out common code from JSON object creation

The JSON objects that are created from the various core MaxScale objects
share a lot of common code. Moving this into a separate files removes the
redundant code.

server/core/json_api.cc

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/json_api.h>
+
+#include <string>
+
+#include <maxscale/config.h>
+#include <jansson.h>
+
+using std::string;
+
+static json_t* self_link(const char* host, const char* endpoint)
+{
+    json_t* self_link = json_object();
+    string links = host;
+    links += endpoint;
+    json_object_set_new(self_link, CN_SELF, json_string(links.c_str()));
+
+    return self_link;
+}
+
+json_t* mxs_json_resource(const char* host, const char* self, json_t* data)
+{
+    ss_dassert(data && (json_is_array(data) || json_is_object(data)));
+    json_t* rval = json_object();
+    json_object_set_new(rval, CN_LINKS, self_link(host, self));
+    json_object_set_new(rval, CN_DATA, data);
+    return rval;
+}
+
+json_t* mxs_json_relationship(const char* host, const char* endpoint)
+{
+    json_t* rel = json_object();
+
+    /** Add the relation self link */
+    json_object_set_new(rel, CN_LINKS, self_link(host, endpoint));
+
+    /** Add empty array of relations */
+    json_object_set_new(rel, CN_DATA, json_array());
+    return rel;
+}
+
+void mxs_json_add_relation(json_t* rel, const char* id, const char* type)
+{
+    json_t* data = json_object_get(rel, CN_DATA);
+    ss_dassert(data && json_is_array(data));
+
+    json_t* obj = json_object();
+    json_object_set_new(obj, CN_ID, json_string(id));
+    json_object_set_new(obj, CN_TYPE, json_string(type));
+    json_array_append(data, obj);
+}