[ES|QL] Separate ENRICH autocomplete routine

[drewdaemon]

Summary

Part of #195418

Gives ENRICH autocomplete logic its own home 🏡

Checklist

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

Identify risks

• As with any refactor, there's a possibility this will introduce a regression in the behavior of commands. However, all automated tests are passing and I have tested the behavior manually and can detect no regression.

[elasticmachine]

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