[Security Solution][Detections] Simplify file download flow

[xcrzx]

Currently, the file download process is unnecessarily complicated from a developer's perspective and includes some redundant steps.

Let's look at the timeline download process, for example. It includes the following steps (simplified):

1. A user clicks Export, and the process kicks off with the download handler (https://github.com/elastic/kibana/blob/master/x-pack/plugins/security_solution/public/timelines/components/open_timeline/edit_timeline_batch_actions.tsx).
2. The download handler calls setState under the hood and sets a bool flag indicating that the download started (https://github.com/elastic/kibana/blob/master/x-pack/plugins/security_solution/public/timelines/components/open_timeline/edit_timeline_actions.tsx)
3. The bool flag gets prop drilled down to the ExportTimeline component
4. The ExportTimeline component reacts to its prop change with useEffect and starts downloading (x-pack/plugins/security_solution/public/timelines/components/open_timeline/export_timeline/export_timeline.tsx)

We can remove state (steps 2 and 3) from the process by downloading files imperatively as soon as the user clicks Export. We also don't need to have the <ExportTimeline /> component and use an imperative downloadBlob method.

Original discussion: #100554 (comment)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)