Support new OAuth 2.0 Client ID Metadata Document functionality

[michaelrsweet]

CUPS should support the new Client ID Metadata Document Internet Draft. The client_id will be a URL ("https://openprinting.github.io/cups/oauth.json") which has an informational URL ("https://openprinting.github.io/cups/oauth.html") describing the client.

The CUPS libraries can look for the client_id_metadata_document_supported boolean in the server metadata to determine whether to use it.