Painless: Add whitelist extensions

[rjernst]

This commit adds a PainlessExtension which may be plugged in via SPI to
add additional classes, methods and members to the painless whitelist on
a per context basis. An example plugin adding and using a whitelist is
also added.

[rjernst]

@debadair I'm really not sure where to link/mention this in the docs. The example plugin I've added here is the "source of truth" for this feature, so I don't really want to add any code to docs, but referencing the file and mentioning it is possible somewhere might be good. Can you suggest a place (or it could be done as a followup)?

[jdconrad]

LGTM. We've worked closely enough on this together that any changes I would have requested have already been made. Thanks for doing this!

[jdconrad]

@rjernst Looked into the failure - noting here. We either need to move the resource files to the same package as Whitelist was moved to or we need to update the WHITELIST_FILES constant to point to the appropriate files.

[jdconrad]

I pushed a fix.

[jdconrad]

@elasticmachine Please test this.

[byoungb]

Any chance on allowing these newly whitelisted Painless extensions in aggregation scripts?

{
  "query": {
    "match_all": {}
  },
  "aggregations": {
    "statuses": {
      "terms": {
        "script": {
          "source": "def e = new ExampleWhitelistedClass(64, 42); return e.publicMember + e.privateMemberAccessor + ExampleWhitelistedClass.CONSTANT;",
          "lang": "painless"
        }
      }
    }
  }
}

[rjernst]

@byoungb
Please ask questions on our forum. We use github for feature requests and confirmed bug reports. Commenting on a closed PR is a likely way to have your problem overlooked.

[byoungb]

I already had but thought it might fit here since this was where the code originated.
https://discuss.elastic.co/t/how-do-you-use-the-examplewhitelistextension-in-terms-aggregations/120551/2