A promise that resolves as soon as the initial negotiation of capabilities has finished.
If it resolves to
true, this peer is now ready to stream media to us. If it resolves to
false, it means this peer can't stream media at all.
requestStream awaits this automatically, so it's never strictly necessary to call this, but it may be useful to know when a peer is "all set" (perhaps to update the UI).
readonly readyToStream: Promise<boolean>;