rieve all Principals of the given type from the authenticated caller's Subject,
 or an empty set if the current caller is not authenticated, or if the specified type
 isn't found in the Subject.
 <p>
 This can be used to retrieve application-specific
 Principals when the platform's representation of the caller uses a different principal type.
 <p>
 The returned Set is not backed by the Subject's internal Principal Set.
 A new Set is created and returned for each method invocation.
 Modifications to the retu