Add support for typing notifications in streams

[neiljp]

The backend for this is now merged via zulip/zulip#17040 and is expected to be in Zulip 4.0

As with the current implementation for PM typing notifications, this is split into extending the event handling, and sending stream (topic) typing status.

For the events we need to use a new client_capabilities value (none of which we set yet), stream_typing_notifications. Without that, we won't receive the events for streams, for backwards compatibility since the event is extended.

See zulip/zulip-mobile#4702 for matching mobile issue, from which some of this detail is taken.