[Fleet] Create new API to manage download_source setting

[criamico]

Summary

First part of #133828

• Create a new API /api/fleet/agent_download_sources around managing agent download sources. It will allow the user to specify the address of a custom registry where the agent can be downloaded from. The default value is artifactory.elastic.co, but the user can change the default to another value, however only one default can exist.
• Adding openapi documentation for the new endpoints - the new docs can be read here
• Adding a new service downloadSourceService
• Adding two methods to AgentPolicyService to handle the new download sources values in agent policies.
• Updating the plugin preconfiguration to set the default value of download source at Fleet startup.

There will be a subsequent PR to update the compilation logic of elastic-agent.yml.

It was decided to not add it to /settings api to maintain a similar structure to outputs, so it was added a new saved object ingest-download-sources.

Endpoints:

• List: GET api/fleet/agent_download_sources - On a fresh start of fleet this should return

{
 "items": [
   {
     "id": "fleet-default-download-source",
     "name": "default",
     "is_default": true,
     "host": "artifactory.elastic.co"
   }
 ],
 "page": 1,
 "perPage": 10000,
 "total": 1
}

• Get single entry: GET /api/fleet/agent_download_sources/{sourceId}

• Create a new entry:

  POST /api/fleet/agent_download_sources
  {
    "name": "host4",
    "host": "http://local-registry",
    "is_default": "true"
  }
  

• Update existing entry:

 PUT /api/fleet/agent_download_sources/{sourceId}
 {
   "name": "host4",
   "host": "http://local-registry",
   "is_default": "false"
 }

• Delete existing entry: DELETE kbn:/api/fleet/agent_download_sources/{sourceId}

Checklist

• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

I don't really have any problems with the code - it's mostly boilerplate API CRUD stuff and lines up with basically every other API resource we have, so all looks good there.

I do have one question about the source_id field. It seems like this is pulled from outputs which define an output_id field, but I'm struggling to understand why we need that, especially since we set the saved objects' id and source_id to the same values on create.

Do you have any context around why we'd need these two attributes rather than just relying on id?

[criamico]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

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• Commit: 65ed344
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Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
fleet	720	723	+3

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
fleet	1309	1340	+31

Page load bundle

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

id	before	after	diff
fleet	95.0KB	95.9KB	+876.0B

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/development-plugin-saved-objects.html#_mappings

id	before	after	diff
ingest-download-sources	-	5	+5

Unknown metric groups

API count

id	before	after	diff
fleet	1436	1467	+31

History

• 💚 Build #52413 succeeded b71abbb
• 💚 Build #52412 succeeded b71abbb
• 💔 Build #52354 failed 712e409
• 💔 Build #52342 failed 0e28819

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

cc @criamico