[Discover] Show unmapped fields when reading from source

[majagrubic]

Summary

Fixes: #91697

This PR shows unmapped fields in discover sidebar when reading fields from source.

Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
- [ ] Documentation was added for features that require explanation or tutorials

• Unit or functional tests were updated or added to match the most common scenarios
  - [ ] Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
  - [ ] Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
  - [ ] If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
  - [ ] This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
  - [ ] This was checked for cross-browser compatibility

For maintainers

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

[wylieconlon]

I've tested this PR in comparison to 7.11 and found that it restores the previous behavior. LGTM

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 645febb

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
discover	426.5KB	426.5KB	+72.0B

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