lavcodec: Adding support for End Trimming in Opus encoder

Adds the end trimming value (duration to be trimmed from the end
of the file due to padding) to the packet's side data. This is
then made use by the muxer to put the value in the container.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>