APIv4 - Add support for HAVING clause

[colemanw]

Overview

Allows the APIv4 query to have a HAVING clause.

Technical Details

Fields referenced in the HAVING clause must be in the SELECT clause as well (HAVING can reference them by their name or alias if used for expressions).

Comments

TODO: Make it look nice in the API Explorer.

[civibot]

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[eileenmcnaughton]

My key criteria for adding additional functions to apiv4 is that they are tested. Our existing test cover gives me some confidence about avoiding breakage