[fleet] Add to_yaml helper

[jeniawhite]

Summary

Adds to_yaml helper to handlebars for fleet server.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

Hi @jeniawhite we added recently a to_json helper #135992 I think it could probably be used in your template (I will be happy to help if needed here) instead of a new to_yaml helper

[kibana-ci]

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id	before	after	diff
apm	14	13	-1
observability	6	5	-1
securitySolution	74	73	-1
total			-3

ESLint disabled line counts

id	before	after	diff
apm	88	85	-3
observability	47	46	-1
synthetics	61	55	-6
ux	10	9	-1
total			-11

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id	before	after	diff
apm	102	98	-4
observability	53	51	-2
securitySolution	499	498	-1
synthetics	67	61	-6
ux	13	12	-1
total			-14

History

• 💔 Build #59099 failed 602b844

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

[nchaulet]

Why do we need to replace undefined by null? I think there is still value to support undefined value if we want some keys to not be in the final yaml no?

[jeniawhite]

Passing objects with undefined key values causes safeDump to throw an exception.

> yaml.safeDump({key: undefined})
Uncaught:
YAMLException: unacceptable kind of an object to dump [object Undefined]
    at writeNode (/Users/evgb/Documents/GitHub/kibana/node_modules/js-yaml/lib/js-yaml/dumper.js:779:13)
    at writeBlockMapping (/Users/evgb/Documents/GitHub/kibana/node_modules/js-yaml/lib/js-yaml/dumper.js:657:10)
    at writeNode (/Users/evgb/Documents/GitHub/kibana/node_modules/js-yaml/lib/js-yaml/dumper.js:750:9)
    at dump (/Users/evgb/Documents/GitHub/kibana/node_modules/js-yaml/lib/js-yaml/dumper.js:840:7)
    at Object.safeDump (/Users/evgb/Documents/GitHub/kibana/node_modules/js-yaml/lib/js-yaml/dumper.js:846:10) {
  reason: 'unacceptable kind of an object to dump [object Undefined]',
  mark: undefined
}

[jeniawhite]

@nchaulet
Sorry for the late response, thank you very much for the feedback.
Due to planning changes, I will close this PR.