[Fleet] Support per agent policy Fleet Server hosts

[nchaulet]

Summary

Related to #137785

That PR allow to override the Fleet server hosts for an agent policy.

To do so we are introducing the following changes:

• the UI to create/edit an agent policy allow to select a Fleet server hosts
• The enrollment Flyout retrieve the correct fleet server for the selected agent policy

UI Changes

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

Is it in scope to change the Fleet Server enrollment Advanced tab to pre-populate the existing Fleet Server host when we select an agent policy?

[juliaElastic]

Tested locally, LGTM 🚀

[Bamieh]

core changes lgtm (an integration_test check hash change)

[kpollich]

Is it in scope to change the Fleet Server enrollment Advanced tab to pre-populate the existing Fleet Server host when we select an agent policy?

This should be covered by the redesign of this landing screen in @criamico's PR here: #144179

[nchaulet]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 1fc5ef2

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
fleet	732	733	+1

Async chunks

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

id	before	after	diff
fleet	855.1KB	855.9KB	+797.0B

Page load bundle

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

id	before	after	diff
fleet	114.5KB	115.0KB	+531.0B

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
ingest-agent-policies	18	19	+1

Unknown metric groups

ESLint disabled in files

id	before	after	diff
osquery	1	2	+1

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
fleet	57	64	+7
osquery	103	108	+5
securitySolution	439	445	+6
total			+20

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	65	72	+7
osquery	104	110	+6
securitySolution	516	522	+6
total			+21

History

• 💔 Build #84343 failed 52b3604
• 💔 Build #84194 failed 41f3900
• 💔 Build #84165 failed 1a10ab0
• 💔 Build #84129 failed a3d7c37
• 💔 Build #84003 failed f9218c9

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

cc @nchaulet