[Fleet] Improve pagination experience on large scale (10k+ agents)

[juliaElastic]

Summary

Fix for #91562

Changing /agents to use point in time request.
Created new API endpoints to open and close PIT request. This is needed so that the same PIT request is reused across many requests on Agent list page.

Remaining work:

• other usages of getAgentsByKuery to be replaced with PIT, e.g. /agent_status call, bulk actions
• fix showUpgradeable filter for >10k agents

WIP: logic works for a few agents, will test with 10k+ with horde.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[kibana-ci]

💚 Build Succeeded

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• Commit: 0b16e5b
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Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
fleet	1300	1307	+7

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	751.9KB	752.3KB	+428.0B

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
fleet	93.1KB	93.6KB	+489.0B

Unknown metric groups

API count

id	before	after	diff
fleet	1427	1434	+7

History

• 💔 Build #51022 failed 12b29f6

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @juliaElastic