Unwrap the multiplexer header from a CDDL message. This can be used to extract the payload from node responses.
@spec plex(binary()) :: map()
iex> Xander.Util.plex(<<0, 0, 0, 0, 1, 2, 0, 3, 97, 98, 99>>) %{payload: <<97, 98, 99>>, protocol_id: <<1, 2>>, size: <<0, 3>>}