Exists API: check if any matching documents exist for a given query

Implements a new Exists API allowing users to do fast exists check on any matched documents for a given query.

This API should be faster then using the Count API as it will:
 - early terminate the search execution once any document is found to exist
 - return the response as soon as the first shard reports matched documents

closes #6995
Call this the "Search Exists API" to make it clear which "exists" functionality it refers to in the docs search dropdown.