GstAudioDecoder class is missing API for adjusting caps on the source pad.

Currently the only way to describe the caps on the source pad of an audio decoder is through

gst_audio_decoder_set_output_format which takes GstAudioInfo as an argument.

As a user I would like to have a possibility to inspect created from audio info caps before base class sends the caps event. For instance user may want to add the features to outgoing caps.