[AI Assistance] Shared DTD service extension manager

[elliette]

Currently the editor side panel is the only place where we listen for extensions registered on DTD, and therefore it currently manages the logic for listening to the service stream.

We need to listen for the sampling extension in our AI assistant as well, therefore we should move the logic for listening to extensions registered on DTD out of the editor panel and into a shared manager which we can use in both places (and potential in the DTD panel as well).