This adds the extension points necessary to enable a user to write a elasticsearch plugin
that can integrate with Shield and add a custom authentication realm. For the most part,
the work here just exposes the existing interfaces we have been using for Realms and
factories to create realms. An additional interface was added to allow for a custom
authentication failure handler to be used. This was needed to support use cases like SSO
and Kerberos where additional headers may need to be sent to the user or a different
HTTP response code would need to be sent.
Relates to elastic/elasticsearch#24
Original commit: elastic/x-pack-elasticsearch@13442e5919
jaymode