Support for IdP Hinting

Add support for IdP Hinting during Discovery. Use cases identified so far include

1. Listing only IdPs that are already registered in the CO
2. Do not list the user's current IdP during account linking
3. Enrollment Flow-specific set of IdPs

There may be multiple technical components to this work, including

1. Plugin based authentication architecture
2. Configuration for determining which IdPs are passed as hints
3. Some sort of linkage between Org Identity login identifiers and the Organization Registry (where Entity IDs can be stored, and populated manually or via a plugin of some sort)

See also AARC-G061 A specification for IdP hinting.