rtmp2: Add llnw auth support to rtmp client
Add support for Limelight CDN (llnw) authentication. Inspired by the ffmpeg implementation of llnw auth.
This commit adds support for llnw auth. I have tested the commit with success to a rtmp server using limelight auth (dacast platform) and verified it looks OK with wireshark. The commit is inspired by the ffmpeg implementation of llnw auth.