Instead of having logic in pion/webrtc and pion/sdp move all logic to
pion/ice. Users of pion/ice don't want to import these packages to use
basic pion/ice functionality.
By default, we should be using unified-plan as the
SDP format of choice. This patch adds a PeerConnection
configuration option to allow the user to specify that
they want plan-b only, unified-plan only, or a special
compatibility mode where a plan-b answer will be generated
IFF a plan-b offer is received.