Create a REST API to describe basic scripting context information

[jdconrad]

Create an internal REST API that will do the following calls:

1. GET a list of available script context names
2. GET information about a specific script context taking in the script context name as a url parameter; this information should include the signature of the execute method, and information about the get* methods

The goal of this API is to be available to pull basic information for creating an auto-complete engine for a Painless editor. A future extension of this API will likely include more information about individual variables available within the script context that will be used to construct automated documentation.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)