Modifying how :muxed is signalled.

Now, instead of using :muxed_audio_video, a list can be supplied to
`content_type` field, indicating that a single CMAF Track carries more
than one multimedia track and their type.

For now, no more information is needed

Author: daniel-jodlos

lib/membrane_cmaf/track.ex

@@ -9,13 +9,15 @@ defmodule Membrane.CMAF.Track do
   ]
   defstruct @enforce_keys
 
+  @type content_type_t() :: :audio | :video
+
   @typedoc """
   Struct describing a single track in Common Media Application Format.
 
   The header field should contain binary contents of a CMAF init file.
   """
   @type t :: %__MODULE__{
-          content_type: :audio | :video | :muxed_audio_video,
+          content_type: content_type_t() | [content_type_t()],
           header: binary
         }
 end