[Discuss] Usage of `yaml` field type / multiline text fields

One of the valid user configuration variable types is `yaml`:
https://github.com/elastic/package-spec/blob/1cd07c67d8c7f6aeda10c5f90fdae0a5b8f95155/versions/1/data_stream/manifest.spec.yml#L58

I do not recall much discussion during the original implementation of this type, but the intention was for Fleet UI to display a code editor with YAML syntax highlighting when this variable type is encountered:
https://github.com/elastic/kibana/blob/77636e73a918b0946ec3fda0b67745663e3498a5/x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/create_package_policy_page/components/package_policy_input_var_field.tsx#L64

We assumed that authors would use this field to allow users to input yaml configuration, i.e. key-value pairs (agnostic of nesting levels, or types of values).

In the Crowdstrike integration, there is one field that expects [multiline text input](https://github.com/elastic/integrations/blob/0457b90a43f4b6a11051226b5c402742902e0a17/packages/crowdstrike/data_stream/fdr/manifest.yml#L115). Originally this field was set to `type: text` which causes Fleet to render a simple one-line input field, which is not a great UX for multiline content, and there was an issue with saving the value due to that. A fix (https://github.com/elastic/integrations/pull/2701) was made by changing it to `type: yaml`. While this works, I'm not sure that `yaml` should be used in this manner for values that are not key-value pairs. Might we consider a different spec for non-yaml, multiline text?

This issue is intended to continue the discussion outside of where the original issue was found.