channels: let subscriber send webxdc updates to channel owner

to push webxdc for channels forward, we should allow subscribers to send updates.

that way, the channel owner will get the full state only, but that is fine for now, as they is the only who can send a webxdc, and can send webxdc known to make sense - already today, things as polls of games make sense - the highscore could be distributed.

this issue is only about the basic update sending.

in a subsequent iteration, we can consider special webxdc that then can distribute the state (for that, the webxdc probably need to know it is a channel owner)

cmp #7678, #7680

cc @Hocuri