[RTC-365] Server notifications

[roznawsk]

Implemented Server Notifications using websockets and asyncio.

I recommend taking a look at the example in the Readme first.
The notification handling occurs as following:

1. Create a Notifier instance
2. Define one or more event handlers using the on_server_notification and on_metrics decorators. Now the Notifier has designated handler for the given message types.
3. Start the notifier (notifier.connect()). This opens a websocket, sends auth request and send subscribe requests but only for the event types which have declared message handlers. This also means that you cannot subscribe to an event type (notifications/ metrics) once the Notifier has started.
4. The event loop runs forever and ends only when the connection is closed from the server or by canceling the notifier task

If you are unfamiliar with asyncio, I recommend taking a look at their docs, for example here: https://docs.python.org/3/library/asyncio-task.html

INB4: I plan to add secure option to both RoomApi and Notifier but not in this PR

[sgfn]

Looks good -- I'm not too familiar with the notifications flow, but as far as I can tell, everything is as it should be. A couple nitpicks only

[LVala]

Suggested change

	You can use it to interact with Jellyfish, managing rooms, peers and components
	You can use it to interact with Jellyfish, manage rooms, peers and components

or ...Jellyfish, enabling the management of rooms..., now it sounds kinda weird.

[LVala]

Nitpick, but I don't like the line break

Suggested change

	async with client.connect(f'ws://{self._server_address}/socket/server/websocket') \
	as websocket:
	address = f'ws://{self._server_address}/socket/server/websocket'
	async with client.connect(address) as websocket:

[LVala]

Does that mean that you cannot add handler after you have been connected? If so, I would mention it somewhere.

[sgfn]

LGTM, cool and good family-friendly version