Let Providers expose a list of registries they trust

[jmandel]

e.g.

a (fixed or discoverable) endpoint like:

GET /.well-known/bb/trusted-registries.json

Returns:

[
  "https://bbplus-static-registry.aws.af.cm",
  "http://blue-button.org"
]

(A use case here: how can we kick off an app launch starting from the provider system? By passing the app enough hint parameters that it can figure out what it needs to know, including, say, which preregistration JWT to pass along. If we have this "trusted registries" list discoverable, then an app just needs to know the URL of the provider and it can figure out the rest.)