graph/users: small optimization for filter by role queries

When filtering by role assignment we now populate the
'appRoleAssignments' property of the returned users when the incoming
request contained and '$expand=appRoleAssignments'. This avoids having
to query the role assignments for all users in the result set a second
time.
The effect of this optimization depends a lot on the actual setup. In
single binary all-in-one installations it is rather small (this it saves
quite a few CPU cycles). In distributed setup it should be larger as it
reduces the number of network round trips to the settings service
significantly.
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