core: split block_t into vlc_frame_t and block_t

This MR try to implement what we discussed during the last Tech meeting: https://code.videolan.org/-/snippets/1313#block_t-and-ancillary

This is prerequisite from #18762 (closed)

This is not a draft: it builds as it is and doesn't require many changes, but it's only the beginning of the split work. If accepted, everyone can fix modules or CORE API one by one. I don't think it's necessary to finish the split for VLC 4.0