[Fleet] Add featureFlag to support standalone fleet server

[nchaulet]

Summary

Related to elastic/fleet-server#2359

Add a new experimental feature that allow to run the Fleet UI with a Fleet Server standalone that do not create a fake agent or checkin.

xpack.fleet.enableExperimental: ['fleetServerStandalone']

That flag will remove the need to get an healthy fleet-server to enroll an agent in the Fleet UI.

cc @michel-laterman @jsoriano

Implementation details

I created a new hook for that to be able to easily change the source of the config here and to enable it if Kibana is running in serverless for example

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 29bdfcf

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
fleet	779	780	+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	930.5KB	930.6KB	+105.0B

Page load bundle

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

id	before	after	diff
fleet	124.5KB	124.7KB	+191.0B

History

• 💔 Build #109633 failed 1bb9f54

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

cc @nchaulet

[jsoriano]

Thanks!