SearchKit - Expose sql functions provided by extensions

[colemanw]

Overview

Allows extensions to contribute SqlFunctions to SearchKit.

Before

When an extension provides a SqlFunction class, it works in the api but is not discoverable in the UI.

After

Discoverable in both SearchKit and the APIv4 Explorer.

[civibot]

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[eileenmcnaughton]

test this please

[colemanw]

@eileenmcnaughton the existing SqlFunctionTest suite covers this.