Deprecate AVStream.side_data and its helpers in favor of the AVStream's
codecpar.coded_side_data.

This will considerably simplify the propagation of global side data to decoders
and from encoders. Instead of having to do it inside packets, it will be
available during init().
Global and frame specific side data will therefore be distinct.
Signed-off-by: James Almer <jamrial@gmail.com>