[Lens]Adds selected field accordion to the fields list

[stratoula]

Summary

Closes #141916

Displays a new section "Selected fields" on the fields list for both the formBased and textBased datasources.

TextBased

• I created 2 accordion groups (available and selected)
• Selected is displayed when there are used fields on the layers
• I added a badge with the fields count on each group (to be aligned with the formBased datasource)

FormBased

• I added one accordion group "Selected fields"
• Selected is displayed when there are used fields on the layers

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• 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)
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 7655d71

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
lens	925	926	+1

Async chunks

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

id	before	after	diff
lens	1.3MB	1.3MB	+2.1KB

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

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[flash1293]

Tested and works as expected, LGTM.

I'm not 100% sure about the interaction for text based languages, but that's something we can iterate on later on as well.

It's definitely helpful to see very easily whether some columns are unmapped for now.