Ability to set security user for Elasticsearch DLS

This is a new processor functionality in the Ingest Node that should be considered for inclusion in Logstash. This allows the setting of the authenticated user info for documents at write time to better enable document level security at query time. For Logstash, I can potentially see this as just another config option(s) in the Elasticsearch output instead of a separate filter.

Additional details on the Ingest processor is here: https://www.elastic.co/guide/en/x-pack/current/field-and-document-access-control.html#set-security-user-processor