Add runtime mappings support via addRuntimeMapping()

[michaelborn]

Implement Elasticsearch runtime mappings via a new addRuntimeMapping() method in the search builder:

searchBuilder.addRuntimeMapping( "hasPricing", {
	"type" : "boolean",
	"script": {
		"source": "doc.containsKey( 'price' )"
	}
} );

Using .addField() ensures the field is returned with the document upon query completion:

searchBuilder.addRuntimeMapping( "hasPricing", ... )
.addField( "hasPricing" );

We can retrieve the result field via the getFields() method:

var documentsWithPricing = searchBuilder.execute()
	.getHits()
	.filter( (document) => document.getFields()["hasPricing"] );

or inlined with the document mento using hit.getDocument( includeFields = true ).

[michaelborn]

Marking this PR as WIP, because I didn't finish testing the actual field retrieval.