This Spark trait is used for mapping a given userName to a set of groups which it belongs to.
This is useful for specifying a common group of admins/developers to provide them admin, modify
and/or view access rights. Based on whether access control checks are enabled using
spark.acls.enable, every time a user tries to access or modify the application, the
SecurityManager gets the corresponding groups a user belongs to from the instance of the groups
mapping provider specified by the entry spark.user.groups.mapping.
Get the groups the user belongs to.
set of groups that the user belongs to. Empty in case of an invalid user.