Serialize tool capabilities JSON without whitespace

fixes #11087

src/convert_json.h

@@ -20,8 +20,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include <json/json.h>
+#include <ostream>
 
 Json::Value fetchJsonValue(const std::string &url,
 		std::vector<std::string> *extra_headers);
 
+void fastWriteJson(const Json::Value &value, std::ostream &to);
+
 std::string fastWriteJson(const Json::Value &value);