mtmd : move helpers to dedicated library (⚠️ breaking change) (#13866)

* mtmd : move helpers to dedicated library

* fix server build

* rm leftover cmakelist code

tools/server/utils.hpp

@@ -6,6 +6,7 @@
 #include "arg.h" // common_remote_get_content
 #include "base64.hpp"
 #include "mtmd.h"
+#include "mtmd-helper.h"
 
 // increase max payload length to allow use of larger context size
 #define CPPHTTPLIB_FORM_URL_ENCODED_PAYLOAD_MAX_LENGTH 1048576