Feature request: add API for event listeners

[Zsailer]

One way to extend Jupyter's Event system would be to enable extensions/plugins of Jupyter Applications to listen for specific events and trigger a callback function. One common use-case would be to enable an extension to emit additional event data that immediately follows the source event.

I think the easiest way to achieve this is to add a simple API to the main EventLogger object, something like:

class EventLogger:
    listeners: dict
    ...
    def add_listener(self, schema_id: str, version: int, listener) -> None:
        ...

    def emit(self, schema_id: str, version: int, data: dict) -> dict:
        ...
        # Event has already been validated and emitted
        for listener in self.listeners[(schema_id, version)]:
            listener(self, schema_id, version, data)

where the signature of the callback is :

def my_listener(logger: EventLogger, schema_id:str, version: int, data: dict) -> None:
    ...

data is the raw (validated) event data.