[Fleet] Fix loading fields for transform destination index template

[nchaulet]

Description

Resolve #175331

We have currently two different approach to build mappings for template and datastreams;

• For datastream, we process all fields field and then put the index template https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/epm/elasticsearch/template/install.ts#L512-L519
• For transform we process the fields file one by one and put the new mappings.

That PR fix that by using processing fields for transform destination index index template the same way we do for datastream index template.

How to test

I added some unit test, to cover the shallow mapping issue.

You can manually try to add some packages with transform and check the mappings looks correct.
I tried a few package myself (endpoint, ti_opencti and mappings looks correct)

[apmmachine]

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[nchaulet]

@chrisberkhout Can you confirm it's the expected behaviour given the two fields files above

[chrisberkhout]

Yes, that's right.

[nchaulet]

/ci

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

LGTM

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: e3d875a

Metrics [docs]

✅ unchanged

History

• 💔 Build #195074 failed 11d3f09
• 💔 Build #195022 failed 43bb7cb

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@elasticmachine merge upstream

cc @nchaulet