Integrate outputs manager into extension app

[ellisonbg]

This integrates the OutputProcessor into the kernel client. The result is that output messages are now written to the server-side ydoc rather than sent to the frontend directly (if the kernel session shows it is a notebook). Most of the logic is in the OutputProcessor and OutputsManager classes which were merged previously. @3coins

[dlqqq]

Thanks for opening this Brian! Can't comment on the core logic since it's outside my scope, but I left some code suggestions below. These are non-blocking, feel free to ignore

[dlqqq]

Does this need to be a trait? It seems like this can be defined as a property:

Suggested change

	outputs_manager = Instance(
	klass=OutputsManager,
	help="An instance of the OutputsManager",
	allow_none=True
	).tag(config=True)
	@property
	def outputs_manager(self) -> OutputsManager:
	return self.settings["outputs_manager"]

This has the added benefit of clearly distinguishing the two attributes:

• outputs_manager_class: a configurable class trait
• outputs_manager: a property on RtcExtensionApp, set by the extension itself

[dlqqq]

Same goes for yroom_manager above.