audio: Add an IEC 61937 payloading library

audio: Add an IEC 61937 payloading library

This can be used by sinks to take compressed formats, correctly payload
these in IEC 61937 frames and feed these to sinks that support
passthrough output over IEC 60958 (S/PDIF) or, in the case of MP3, over
Bluetooth.

Initial implementation includes AC3, E-AC3, MPEG-1, MPEG-2 (non-AAC),
and DTS (type-I/II/II) payloading. More formats can be added as needed.

API: gst_audio_iec61937_frame_size()
API: gst_audio_iec61937_payload()

https://bugzilla.gnome.org/show_bug.cgi?id=642730