Expose op:sample-by in Node Client API

[llinggit]

As task in bugtrack, https://bugtrack.marklogic.com/55089
A sample query as below

xquery version "1.0-ml";
import module namespace op="http://marklogic.com/optic"
at "/MarkLogic/optic.xqy";
op:from-view("opticUnitTest", "musician")
=> op:sample-by(map:map()=>map:with("limit", 2))
=> op:result()

[ehennum]

I'm guessing that the SJS unit test for sampleBy() can be adapted to provide a Node.js API unit test.