[Fleet] Add new APIs and preconfiguration for Fleet server hosts per policy

[nchaulet]

Summary

Related to #137785

That PR add a new saved object fleet-fleet-server-hosts that will allow to get per agent policy Fleet server host.

Add new CRUD Apis to manage Fleet server hosts:

/api/fleet/fleet_server_hosts
/api/fleet/fleet_server_hosts/{itemId}

Add a new preconfiguration key in kibana config yo manage this hosts:

xpack.fleet.fleetServerHosts:
  - id: fleet123
    name: 'TEST 456'
    is_default: true
    host_urls: ['http://fleetserver:123']

The old kibana config key xpack.fleet.agents.fleet_server.hosts will create a new Fleet server hosts saved object too, to not introduce a breaking change.

Not done in that PR: the saved object is currently not used to build agent policies this will come in a following PR with the UI work.

Todo

• Add basic API integrations
• Add API doc

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[criamico]

LGTM

[nchaulet]

@elasticmachine merge upstream

[nchaulet]

@elasticmachine merge upstream

[jbudz]

packages/kbn-test

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 53e5362

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
fleet	770	772	+2

Saved Objects .kibana field count

Every field in each saved object type adds overhead to Elasticsearch. Kibana needs to keep the total field count below Elasticsearch's default limit of 1000 fields. Only specify field mappings for the fields you wish to search on or query. See https://www.elastic.co/guide/en/kibana/master/saved-objects-service.html#_mappings

id	before	after	diff
fleet-fleet-server-host	-	5	+5

History

• 💔 Build #78851 failed dbf576e
• 💚 Build #78798 succeeded c21d12b
• 💔 Build #78743 failed c86f5ab
• 💔 Build #78694 failed efb1725
• 💚 Build #78498 succeeded 1a1521d

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

cc @nchaulet