This commit adds the endpoint options required to control
Advanced Codec Negotiation.

incoming_offer_codec_prefs
outgoing_offer_codec_prefs
incoming_answer_codec_prefs
outgoing_answer_codec_prefs

The documentation may need tweaking and some additional edits
added, especially for the "answer" prefs.  That'll be handled
when things finalize.

This commit is safe to merge as it doens't alter any existing
functionality nor does it alter the previous codec negotiation
work which may now be obsolete.

Change-Id: I920ba925d7dd36430dfd2ebd9d82d23f123d0e11