Allow callbacks after append and stream

[ahuang11]

Motivated by holoviz/lumen#544

I'd like to trigger a callback after every new message, but it's hard to do right now because of the various ways to append a message.

Initially I wanted to just super append but it doesn't cover the case of streaming (e.g. the contents were empty for the callback trigger). Here, I wait for all the streaming to be done before triggering append callback.

[philippjfr]

Hmm, should we not simply be waiting to trigger the regular callback until it's done streaming?

[ahuang11]

to trigger the regular callback until it's done streaming?

If I understand correctly, the regular callback triggers the streaming, i.e.

1. user sends a message
2. "regular callback" runs the response callback that could stream output
3. append_callback runs after the regular callback