SQL: maybe register its own scripting engine

[elasticmachine]

Original comment by @costin:

Worth investigating whether we can move away from generating Painless code. Or rather use Painless directly, as a 'trusted' component to get some nice optimizations in place and avoid script parsing/compilation...

[elasticmachine]

Original comment by @nik9000:

This isn't about an optimization so much as about not building painless scripts with strings which feels super error prone.

[elasticmachine]

Original comment by @costin:

Another example of where this will help; Random function uses org.elasticsearch.common.Randomness class while in Painless we're forced to use java.util.Random.