[Cases] Refactor plugin cases "Add to new case" flyout show and hide logic 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.

To detach the "Add to new case" from timelines some code refactor is required in the display and hide flyout logic.

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

Acceptance criteria:

• The timelines plugin doesn't handle opening and closing the cases flyout
• Security and Observability are updated to create and handle the Cases context out of timeline scope.
• Tests are updated in the timelines and cases plugin
• Old code is kept for backwards compatibility (to be removed in following issues)

Existing setup:

Suggested setup:

Existing contexts setup:

Suggested contexts setup:

[elasticmachine]

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

[elasticmachine]

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