Changelog: This patch introduces a mechanism similar to
`Client::add_event_handler` and `Client::add_room_event_handler`
but with a reactive programming pattern. This patch adds
`Client::observe_events` and `Client::observe_room_events`.
```rust
// Get an observer.
let observer =
client.observe_events::<SyncRoomMessageEvent, (Room, Vec<Action>)>();
// Subscribe to the observer.
let mut subscriber = observer.subscribe();
// Use the subscriber as a `Stream`.
let (message_event, (room, push_actions)) = subscriber.next().await.unwrap();
```
When calling `observe_events`, one has to specify the type of event
(in the example, `SyncRoomMessageEvent`) and a context (in the example,
`(Room, Vec<Action>)`, respectively for the room and the push actions).
Ivan Enderlin
6cef7f20c5