Command functions should be able to return an updated config

[keller-mark]

Chat plugin should be able to both return a string and also return an updated config dict (optionally).

Need to update the python APIs to enable this (would be similar to the on_config_change inputs/outputs and behavior).

https://github.com/vitessce/vitessce-python/blob/main/docs/notebooks/widget_plugin_custom.ipynb

[keller-mark]

Or maybe the command function should just receive a reference to the widget instance?