Support for Socket.IO v4.1.0 inter-server communication (`serverSideEmit`)

[Kakadus]

Is your feature request related to a problem? Please describe.
In a multi-server scenario with a message queue such as redis or rabittmq, it is important to be able to send messages between servers.
That is why socket.io 4.1 introduced the serverSideEmit function: dzad/socket.io@491a9a0. This seems to be missing from python-socketio though.

Ideally, parameters like room or to can limit the servers the message is sent to.

Describe the solution you'd like
A sio.server_side_emit(..., to=..., room=...) function and self.server_side_emit(..., to=..., room=...) method to do inter-server communication.

Describe alternatives you've considered

• adding clients between servers manually. This is not really feasible with dynamically scaled servers.
• configuring the client manager manually. This seems not feasible either.

Additional context

# server A
await sio.server_side_emit("hello", "world", room="test")


# server B
@sio.on("hello")
async def server_notifications(sid, message):
    print(message)  # should print "world" if server is in room "test"

[miguelgrinberg]

This is not a top priority for me at this time, but I would be open to add this if someone contributes it.

But, I do not quite follow some of your requests:

Ideally, parameters like room or to can limit the servers the message is sent to.

What does it mean to send a server side emit to a room? If there are clients in that room, do they get the message? How do you add a server to a room? What do you pass in the to argument to reference a specific server? I don't see what is the point of this, or what problem does it solve.