Jack Conradson b57af6c401 Add a Painless Context REST API (#39382)
This PR adds an internal REST API for querying context information about 
Painless whitelists.

Commands include the following:
GET /_scripts/painless/_context -- retrieves a list of contexts
GET /_scripts/painless/_context?context=%name% retrieves all available 
information about the API for this specific context
2019-03-14 12:42:12 -07:00
..