Support SEI side_data parsing

Created by: choice17

Overview

Thank you for developing/maintaining pyav! I would like to make a feature request to support sei side data parsing for h264 hevc.

Existing FFmpeg API

FFmpeg is now support SEI type side_data for 4.3 https://github.com/FFmpeg/FFmpeg/blame/master/libavutil/frame.h#L187

However, there is no such enum / definition for SEI.

https://github.com/PyAV-Org/PyAV/blob/main/av/sidedata/sidedata.pyx#L18

Expected PyAV API

Example:
Refer to motion vector side data example in
https://github.com/PyAV-Org/PyAV/blob/main/tests/test_decode.py#L53

def frame_iter(video):
    #streams = [s for s in video.streams if s.type == b'video']
    #print(streams)
    streams = video.streams.video
    for packet in video.demux(streams):
        print(packet, packet.buffer_size, packet.buffer_ptr,dir(packet))
        for frame in packet.decode():
            yield frame


video = av.open(sys.argv[1])
print(video, video.streams.video)

frames = frame_iter(video)
for frame in frames:
	sei = frame.side_data.get(av.AV_FRAME_DATA_SEI_UNREGISTERED)
        print(sei.data)

• FFmpeg:

version 4.3.X