Feature Request: shutdown hook in BubHookSpecs

[emliunix]

For usecases like to close plugin held resources, eg. db connections.

So I'm experimenting write my own SQLite backed tape store. And I found bub chat won't exit on Ctrl-D, cause the db connection is still alive.

The AsyncTapeStore interface doesn't have a close or shutdown method. And I think it might be more general to have a shutdown or even async initialize (current init relies on __init__, which is sync) at HookSpec for full lifecycle awareness.