[Fleet] Honor source_mode defined in a package datastream when installing it

[nchaulet]

Description

Closes #141211

When installing a package we should honor the value of source_mode if a package datastream define it.

That PR does that.

The experiment datastream UI in the package policy editor need to be udpated too, this will come in a following PR.

Testing

I added some unit test to validate how generate the mappings, and you can test locally by adding this to a datastream manifest, I tested it with nginx stubstatus and it seems to work well

elasticsearch:
  source_mode: "synthetic"

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[hop-dev]

@nchaulet this may be useful for testing - I previously added x-pack/test/fleet_api_integration/apis/fixtures/test_packages/non_epr_fields/1.0.0 which has a stream with each of the different values for source_mode

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: a95fa89

Metrics [docs]

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	59	65	+6
osquery	109	115	+6
securitySolution	443	449	+6
total			+20

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	68	74	+6
osquery	110	117	+7
securitySolution	520	526	+6
total			+21

History

• 💔 Build #92753 failed 27e6fabe6c495de2faebff13a698e237d58d39ba

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

cc @nchaulet

[ruflin]

@nchaulet Happy to test this on my end. Let me know when the follow up PR also made it in.