[Cases] Refactor "Add to case" timeline action button to move it out of timelines.

[academo]

See epic #123183

The existing integration between alert timelines and cases consists of a series of hooks, contexts and callbacks that are not entirely reusable for other plugins and that require the timelines plugin to keep logic to track of open flyouts and to update cases to attach information to them.

Currently the "Add to case" and "Add to new case" action buttons are provided by the timelines plugin but are a wrapper of the cases create flyout to help handling the show/hide flyout logic.

Once #124524 is completed, there's no need for the timelines plugin to re-export this functionality from cases and should be moved to the cases plugin.

Current setup

Desired setup

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[cnasikas]

@academo Should we close the issue?