Add Support for new AI events: assistant_thread_started and assistant_thread_context_changed

[anothrNick]

Description

With the introduction of AI supported apps in Slack, the addition of structs to support the relevant inner events are needed.

(Optional) Slack's documentation

Slack AI Assistant flow
assistant_thread_started
assistant_thread_context_changed

If work has not already begun on this, I'm happy to submit a PR for this work. Just want to be sure there is no duplicate effort.

[MattDavisRV]

I created a PR for these changes, it can be found here

[ahmednexum]

Yes please this is very important :) Thank you @MattDavisRV