audiodecoder: add API for setting caps on the source pad

This patch adds API in the audio decoder base class for setting the arbitrary
caps on the source pad.  Previously only caps converted from audio info were
possible.  This is particularly useful when subclass wants to set caps features
for audio decoder producing metadata.
2 jobs for master in 2 minutes and 25 seconds (queued for 4 seconds)
Status Name Job ID Coverage
passed manifest #44303


passed build fedora x86_64 #44304