[UII] Allow free-form text in KQL searches (again)

[jen-huang]

Summary

Part of https://github.com/elastic/kibana/issues/171425.

This PR allows parsed KQL expressions that don't have a key/field. For example, the follow query expressions are now allowed when searching agents:

last_checkin_message.keyword : "Running" and Development

macbook

"8.15.0" and tags : "Development" 

This PR also enables the flag enableStrictKQLValidation now that both free-form text expressions are allowed and validation remains on expressions which specify a field.

Checklist

Delete any items that are not applicable to this PR.

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