[SPARK-3250] Implement Gap Sampling optimization for random sampling

More efficient sampling, based on Gap Sampling optimization:
http://erikerlandson.github.io/blog/2014/09/11/faster-random-samples-with-gap-sampling/

can you change the comment to use javadoc style? e.g.
```scala
/**
 * Default gap sampling maximum.
 * ...
 */
````