[Cases] Refactor plugin cases "Attach to existing case" (select case) modal logic out of the timelines plugin

[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.

To detach the "Attach to existing case" from timelines some code refactor is required in the display and hide the modal logic.

Currently the timelines plugin has the logic to open and close the cases modal, this logic should be moved to the cases plugin control (see EPIC).

Acceptance criteria:

• The timelines plugin doesn't handle opening and closing the select cases modal
• Tests are updated in the timelines and cases plugin
• Old code is kept for backwards compatibility (to be removed in following issues)

[elasticmachine]

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

[elasticmachine]

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