handle extmap directionality attribute

[fippo]

from https://github.com/fippo/adapter/issues/44

not entirely sure if adapter is affected by https://bugs.chromium.org/p/webrtc/issues/detail?id=7477 so need to check. Edge doesn't do ssrc-audio-level yet so there is no interop issue. Still the sdp package should parse extmap properly with direction and the answer should take this into account