Add `class_id` to client schema

[wilsonianb]

https://datatracker.ietf.org/doc/html/draft-ietf-gnap-core-protocol#section-2.3

class_id (string): An identifier string that the AS can use to
identify the client software comprising this client instance. The
contents and format of this field are up to the AS. OPTIONAL.

I'm wondering if it would be useful at to specify something like rafiki there to allow special handling of clients that are Open Payments servers.