Make MDProcessType JSON serializable

src/enums/MDValueEnumInterface.php

@@ -55,4 +55,11 @@ interface MDValueEnumInterface {
      * @return string
      */
     public function getTledName(MDTlLoader $tlLoader):string;
+
+    /**
+     * Provides the option to serialize as a string during json_encode().
+     *
+     * @return string
+     */
+    public function jsonSerialize():string;
 }