[Cases][Epic] Improve how plugins consumes the "attach to a case" functionality

[cnasikas]

Cases plugin provides a flyout and a modal for other plugins to be able to attach data to a case. At the moment, plugins need to keep the state of opening/closing the modal or the flyout and know the internals of how to attach the data to the cases. Cases should provide an out-of-the-box solution where plugins with minimal configuration can use the feature of attaching data to a case. Cases should:

• Keep the state opening/closing of the flyout or the modal internally
• Provide attach to case buttons that can be used in a context menu or a custom UI
• Allow plugins to pass the data they want to attach to a case.

Proposal by @semd

Cases can export an add-to-case context where the consumer plugins can use along with the add-to-case buttons. For example:

<AddToCasesContext>
   <ConsumerPlugin>
        ...
   		<AddToCase attachments=[ { alerts: [{ id: 'test-id', index: '.test-index' }] }] />
   </ConsumerPlugin>
</AddToCasesContext>

Issues:

• [Cases] Refactor timelines plugin "add to new case" internal logic.  #124359
• [Cases] Refactor plugin cases "Add to new case" flyout show and hide logic out of timelines #124524
• [Cases] Refactor plugin cases "Attach to existing case" (select case) modal logic out of the timelines plugin #125782
• [Cases] Delete dead code getAddToCasePopover #125658
• Refactor "Add to new case" timeline action button to move it out of timelines. #126069
• Refactor "Add to existing case" timeline action button to move it out of timelines. #126070
• [Cases] Cleanup: Remove old APIs after refactors (once completed and merged) #126264
• [Cases] Cleanup: Delete dead code for timelines add to new case flyout display and hide logic #125761
• [Cases] Write cases plugin third party plugins integration documentation #125780
• [Cases] Security Solution and Observability should use CasesContext higher in the render tree. #127120
• [Cases] Remove deprecated APIs from the cases plugin.ts API #127124

Meta: #123310

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

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