CIBA Customization Extensibility Points

[josephdecock]

This adds extensibility points to CIBA to allow for custom request and response parameters.

• Adds a new custom backchannel authentication request validator hook, similar to AddCustomAuthorizeRequestValidator and AddCustomTokenRequestValidator
• Adds a context dictionary to the ciba validation result, which then feeds into similar context dictionaries in the response to the client, notification to the user, and record in the database

[josephdecock]

This PR began as #1361, but that PR didn't get merged for a long time, so I rebased it and created a new PR here. @gislikonrad - thanks for raising this issue and submitting the PR back in the summer! Do you think this new approach will meet your needs?

[brockallen]

Two thoughts:

1. I like the concept that it's a "context", but it's also "custom". So maybe a better name to indicate that it's custom? The token EP has "Custom", but maybe "CustomContext"?

2. We don't have any tests for this new feature. We should (both for the new custom validator extensibility point, but also emitting custom results that flatten into the response payload).

[gislikonrad]

This would work for us at our organization. I just had some comments on the naming of the new properties in the backchannel authentication models and contexts.