oauth: What metadata do we require for registration?

[aragilar]

client_name and grant_types are the obvious ones, see more at https://openid.net/specs/openid-connect-registration-1_0.html#ClientMetadata.

See #22 for context.

[mbtaylor]

I wonder if application_type would be useful when processing a registration? But I'm on the other side of that interaction, so I may be wrong.