rtpmp4apay: add "config-interval" property to multiplex aac audio config data into resulting data stream

Submitted by wul..@..il.com

Link to original bug (#629545)

Description

Freature Request: rtpmp4apay element GStreamer

Up till now with the help of rtpmp4apay one is able to stream aac audio data compliant to RFC 3016. But it's still necessary to tell the backend
the config parameter (StreamMuxConfig element) by means of SDP for example in order to listen to the music at the receiver side. This request
is about adding the feature of multiplexing aac audio config data into the resulting data stream.