Add EXT-X-STREAM-INF for audio only track

[kingdomcoding]

A fix for audio- only streams.

For cases like below where the video pad of the hls_sink_bin has nothing passed into it:

child(:rtmp_source, %Membrane.RTMP.SourceBin{client_ref: client_ref})
|> via_out(:audio)
|> child(:rtmp_in_aac_parser, Membrane.AAC.Parser)
|> child(:rtmp_in_aac_decoder, Membrane.AAC.FDK.Decoder),
child(
  :hls_sink_bin,
  %Membrane.HTTPAdaptiveStream.SinkBin{
    manifest_name: manifest_name,
    manifest_module: Membrane.HTTPAdaptiveStream.HLS,
    storage: %Membrane.HTTPAdaptiveStream.Storages.FileStorage{
      directory: directory
    },
    hls_mode: hls_mode,
    mp4_parameters_in_band?: true,
    target_window_duration: Membrane.Time.seconds(20)
  }
),
# AUDIO BUILDER
get_child(:rtmp_in_aac_decoder)
|> child(:hls_out_aac_encoder, Membrane.AAC.FDK.Encoder)
|> via_in(Pad.ref(:input, :audio),
  options: [encoding: :AAC, segment_duration: Membrane.Time.milliseconds(2000)]
)
|> get_child(:hls_sink_bin),

# FAKE VIDEO BUILDER
get_child(:rtmp_source) |> via_out(:video)
|> child(:fake_sink, Membrane.Fake.Sink.Buffers)

[Karolk99]

Great work! Thanks for your valuable contribution. I'll merge this after adding a test for the audio stream only.