lamemp3enc: implement sinkpad get_caps() function to proxy rate and channels restrict...

lamemp3enc: implement sinkpad get_caps() function to proxy rate and channels restrictions from downstream

The element downstream of mp3enc might only accept certain sample rates or channels,
make sure we relay any restrictions that do exist to upstream when it does a
get_caps() on the sink pad. That way upstream elements like audioresample or
audioconvert can pick a sample rate / channel configuration that will be accepted,
instead of just negotiating to the highest, which might then be rejected.

https://bugzilla.gnome.org/show_bug.cgi?id=641151