…astic#197744)

## Summary

This PR begins the refactor described in
elastic#195418.

The autocomplete engine now delegates to command-specific routines
attached to the command definitions for `KEEP`, `DROP`, and `SORT`.

The naming of `getFieldsFor` has been broadened to `getColumnsFor`
because the response from Elasticsearch can contain variables as well as
fields, depending on the query that is used to fetch the columns.

No user-facing behavior should have changed.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
(cherry picked from commit 11ae6a5)

…SORT` autocomplete routines (#197744) (#198159)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[ES|QL] separate `KEEP`, `DROP`, and
`SORT` autocomplete routines
(#197744)](#197744)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Drew
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separate `KEEP`, `DROP`, and `SORT` autocomplete routines
(#197744)\n\n## Summary\r\n\r\nThis PR begins the refactor described
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separate `KEEP`, `DROP`, and `SORT` autocomplete
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separate `KEEP`, `DROP`, and `SORT` autocomplete routines
(#197744)\n\n## Summary\r\n\r\nThis PR begins the refactor described
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autocomplete engine now delegates to command-specific
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separate `KEEP`, `DROP`, and `SORT` autocomplete routines
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autocomplete engine now delegates to command-specific
routines\r\nattached to the command definitions for `KEEP`, `DROP`, and
`SORT`.\r\n\r\nThe naming of `getFieldsFor` has been broadened to
`getColumnsFor`\r\nbecause the response from Elasticsearch can contain
variables as well as\r\nfields, depending on the query that is used to
fetch the columns.\r\n\r\nNo user-facing behavior should have
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BACKPORT-->

Co-authored-by: Drew Tate <drew.tate@elastic.co>
Co-authored-by: Stratoula Kalafateli <efstratia.kalafateli@elastic.co>