Add constants to the API for various keys

Jakarta Authorization specifies various string based keys, e.g. for system properties and for context handlers. Those keys however do not have constants in the API yet. We should add them for both ease of use and consistency.

Start by reviewing the Jakarta Authorization specification sections covering system-property and context-handler keys, then locate the public API package where related constants belong. The issue names no files or tests; done means the specified keys have consistent public constants with names and values verified against the specification.

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