Scripting: Add script engine for lucene expressions

Adds support for new "expression" script engine.

This is backed by lucene expressions.  These are javascript expressions, which can only access numeric fielddata, parameters, and _score.  They can only be used for searches (not document updates).

closes #6818
It should be optional, but we should make sure we package it as part of our distro. In our script service we already support optional mvel and groovy, so thats easy, similar logic.

Its less about the size, its about the fact that those are 2 very popular libraries, and almost certainly will cause clashes when someone wants to use a NodeClient / TransportClient

In order to include it in our distro, make sure to add it in the deb/rpm logic in the pom.xml (with all deps), and in src/main/assemblies/common-bin, then run the package and check tar.gz includes all the deps, as well as the deb and rpm