Prefer decorators for emitting awareness events

[dlqqq]

          @jzhang20133 Thank you opening this PR! I left a comment below, but David Brochart's review covers the areas of feedback blocking this PR from being merged. 😁 

All I would like to add is that it's possible to emit events through a decorator interface. Emitting events imperatively (as done in this PR) is less preferable, since it's really easy for a developer to accidentally delete the line emitting the event. Using a decorator also allows for this feature to be added without any changes to the method definition, which I take as a sign of clean code.

To implement a decorator interface, you would want to define an @emit_awareness_event decorator that looks something like:

def emit_awareness_event(action: str, message: Optional[str] = None):
    def decorator(method):
        async def method_stub(self, *args, **kwargs):
            return_value = await method(self, *args, **kwargs)
            data = { "action": action, ... }
            self.event_logger.emit(..., data=data)
            return return_value
        return method_stub
    return decorator

After doing so, the method declarations would look like:

    @emit_awareness_event("join")
    async def open(self, room_id):
        ...

    @emit_awareness_event("leave")
    def on_close(self) -> None:
        ...

Let me know if this all sounds like a good idea for future work. If so, I can track this in a new issue so as to not block this PR. Thanks! 🤗

Originally posted by @dlqqq in #246 (review)