Factor out decoder manager

[ronen]

[Another step towards #74]

Created a PeerDecoderManager (I'm open to a better name!) that handles keeping track of the currently connected peers, dispatching each packet to the appropriate decoder, etc. factored all that out of Attendants.

It provides some callbacks that are set by Attendants:

• on_peer_added(key) -- forces an update
• on_first_frame((key, media_type)) -- forces an update if the media type is screen, this fixes the hack of d760779b
• get_video_canvas_id(key) called to get DOM ID of the canvas element into which video should be rendered
• get_share_canvas_id(key) called to get DOM ID of the canvas element into which screen share should be rendered

In the future, could add on_last_frame((key, media_type)) and on_peer_removed(key)) to handle a peer stopping audio, video or screen share or a peer disconnecting entirely, if/when there's protocol to detect those.

[darioalessandro]

@ronen we mentioned in our reddit announcement, I hope that is ok with you: https://www.reddit.com/r/rust/comments/14vwxfh/we_just_released_our_rust_webtransport/?utm_source=share&utm_medium=web2x&context=3

Thanks for helping us to shape up a proper architecture!!

[ronen]

@ronen we mentioned in our reddit announcement, I hope that is ok with you:

That's fine, thanks for the shout out.

More to come...

[darioalessandro]

still looking at this, sorry for the delay!

[darioalessandro]

Code looks good, I'll just test tonight to make sure that everything still works