Add an SEI message package for AVC and HEVC

Add an SEI message package for AVC and HEVC: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). The library can already pull SEI (Supplementary Enhancement Information) NAL units out of a stream, but there is no shared, codec-aware way to parse their contents. I'd…