GroupMappingServiceProvider as a Knox Provider

Introduce a specific provider for integrating the Hadoop GroupMappingServiceProvider for looking up group membership of the authenticated (asserted) identity.

This will allow customers' existing investments in the Hadoop hook to be leveraged within Knox and used within the access control policy enforcement at the perimeter.

This mechanism will likely be mutual exclusive of the Shiro provider group support - though there may be use cases where they could supplement each other.