Context

There is a target property/attribute for getting user groups from the IdP in GroupsAuthorizationGenerator

The problem is it's not configurable at the moment, causing the inconvenience and our customers have to provide a custom implementation for AuthorizationGenerator just for the attribute name.

Proposed solution

• Make the attribute name configurable (could place the config in 
  GroupsAuthorizationGenerator class, like "mappings" attribute)
• The default attribute name should be "groups" (as it's now)