[Fleet] Improve OpenAPI spec for agent bulk actions

[juliaElastic]

Improve OpenAPI spec for agent bulk actions:

• bulk_reassign
• bulk_enroll
• bulk_upgrade

All of these endpoints support an agents parameter which can be either a list of agent ids or a query string.
Update the documentation so that it is clearer what is supported.

E.g. https://petstore.swagger.io/?url=https://raw.githubusercontent.com/elastic/kibana/main/x-pack/plugins/fleet/common/openapi/bundled.json#/default/bulk-reassign-agents

Some examples by triggering from UI:

Filtering on 2 agent policies:
POST http://localhost:5620/api/fleet/agents/bulk_reassign
{"policy_id":"e9d53200-f7b4-11ec-a9a9-0d6b33170dda","agents":" fleet-agents.policy_id : (\"9a00e890-f783-11ec-a9a9-0d6b33170dda\" or \"e9d53200-f7b4-11ec-a9a9-0d6b33170dda\")"}

No filters:
POST http://localhost:5620/api/fleet/agents/bulk_upgrade
{"version":"8.2.1","agents":"","rollout_duration_seconds":3600,"start_time":"2022-06-29T14:13:24.649Z"}

Filter on Updating only:
POST http://localhost:5620/api/fleet/agents/bulk_unenroll
{"agents":"((upgrade_started_at:*) and not (upgraded_at:*)) or (not (last_checkin:*)) or (unenrollment_started_at:*)","revoke":false}

Originally posted by @juliaElastic in #135425 (comment)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

@kpollich created this issue to improve OpenAPI spec.