Auto load/unload custom udf function for hive cli and hiveserver2

when adding custom functions used in hiveserver2, the most method is re-build the hive source code, re-dist and restart hiveserver2. This way will produce big cost for service user and cluster manager. The best way, in my opinion, the custom udf should be like a plugin to the hiveserver2 and hive cli, and users can add and remove at run-time, especially for hiveserver2.