add webrtc-audio-coding public library

This MR is a superset of !7 (merged)

@arun : so I tried syncing webrtc-audio-processing with current version but it turned out to be a nightmare: code has been heavily refactored, files moved around, new 3rd party deps, etc. I spent lots of time on this and then gave up. Then I tried just adding the iSAC files I need for my gst elements using the same old revision as the one we currently use in webrtc-audio-processing and managed to make it work quite easily. So would you consider merge this so we can add the encoder and decoder elements to GStreamer?