feat(sdk): Implement Client::observe_events and Client::observe_room_events

[Hywan]

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.

// 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).

Read the documentation of the ObservableEventHandler and
EventHandlerSubscriber to learn more about how it works.

In order to achieve that, it was necessary to implement EventHandlerContext
on tuples where each part implements EventHandlerContext. Why? Because,
while EventHandler is implemented for FnOnce() with 0 to 8 arguments
(e.g. with 2 arguments client.add_event_handler(|ev: E, ctx1: C1, ctx2: C2| { … })),
it is not possible to have “variadic” generic parameters. Example, it's not possible
to write client.observe_events::<E, C1, C2>(). The trick is to use tuples (!), like:
client.observe_events::<E, (C1, C2)>(): the type of observe_events remains
“constant”:

pub fn observe_events<Ev, Ctx>(&self) -> ObservableEventHandler<(Ev, Ctx)>
where
    Ev: SyncEvent + DeserializeOwned + Send + Sync + 'static,
    Ctx: EventHandlerContext + Send + Sync + 'static,
{
   …
}

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.93%. Comparing base (36b96cc) to head (e2b1818).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_handler/mod.rs	76.47%	4 Missing ⚠️
crates/matrix-sdk/src/event_handler/context.rs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4253      +/-   ##
==========================================
+ Coverage   84.90%   84.93%   +0.02%     
==========================================
  Files         274      274              
  Lines       29761    29795      +34     
==========================================
+ Hits        25270    25307      +37     
+ Misses       4491     4488       -3     

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[jmartinesp]

Thanks, the changes LGTM in general since I assume it's correct, but maybe a second pair of more experienced eyes wouldn't hurt since I'm not familiar with some parts of it.

[poljar]

Very nice. Quite the clever trick on how to make this pattern generic, great work.

I left some small nits about the documentation but otherwise this is good to go.