[Fleet] Add a find orphaned integration policies API

[jen-huang]

Describe the feature:

Similar to #121887. We've had several reports where the root cause is that somehow an integration policy became orphaned. That is, its parent agent policy does not exist.

Proposal

POST /internal/fleet/find_orphaned_policies

This would return a list of all orphaned policies - but not delete them yet. I think subsequent deletion should be up to the user as there might be information they want to capture from the policies before they are lost. It would then be trivial to plug in the policy IDs into the deletion API:

$ curl -X POST -u USERNAME:PASSWORD <KBN_HOST>/api/fleet/package_policies/delete -H 'kbn-xsrf: xyz' -H "Content-Type: application/json" -d '{"packagePolicyIds": ["some-id-1", "some-id-2"], "force": true}'

Edit: the above curl doesn't actually work, due to #129683. The workaround is to delete the saved objects directly:

DELETE .kibana/_doc/ingest-package-policies:<INTEGRATION_POLICY_ID>

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

added API in this pr as part of spacetime project: #131697

[jen-huang]

Closing as implemented in #131697.