SearchKit - Fix arithmetic in WHERE clause

[colemanw]

Overview

Permit SqlEquation in APIv4 WHERE clauses so it works in SearchKit.
Fixes dev/core#3845

Before

Trying to use arithmetic in the WHERE clause will fail.

After

Works + test.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[demeritcowboy]

Merging based on lab ticket review by reporter.