Closes #1548. Declare PrincipalCollection interface to be intended to be immutable; provide an immutable implementation including a builder class; deprecate the mutable implementation; use the immutable one in SimpleAuthenticationInfo for merging.

Background: Merging AuthenticationInfo and the contained PrincipalCollections previously lead to one of the involved PrincipalCollections be selected by undefined means and being mutated, propagating the changes to other callers that did not expect such changes, including the authentication cache.

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