add/sub done but untested, mod remains. this commit also adds a message handler inside the rtp endpoint so it can properly flush streams, setup jitter buffering, and honor uuid_debug_audio requests

command

     - only one to one rtcp is supported for currently
     - jitter stats and loss stats are not real

adding media_gateway_cmd_handler.c for all commands related  logic/APIs.
adding media_gateway_packages.c to have list of all supported packages.
adding media_gateway_utils.c for all media gateway utility APIs