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

[ahmwanes]

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

[surbhiijain]

Can you please make a new release with these updates? I see that everything was added in for assistants in early November but there wasn't a new release