Add a `Parameters` bag to the authentication properties that allow
passing arbitrary parameters to an authentication handler. These values
are not intended for serialization of persistence, only for flowing data
between call sites.
Also make existing `Items` collection helpers protected to allow them to
be reused in subclasses, make string-based helpers public as a public
way to work with the collection, and add helper methods to interact with
the `Parameters` dictionary.
Gert Driesen
Patrick Westerhoff