Add cnf claim to Example 4b (per issue #320)

[bc-pi]

https://drafts.oauth.net/oauth-selective-disclosure-jwt/bc-cnf-to-ex4b/draft-ietf-oauth-selective-disclosure-jwt.html#appendix-A.4

per issue #320

[bc-pi]

description of example 4b also needs to be updated to say the example contains key binding

It does already say "Key Binding is applied using the Holder's public key passed in a cnf claim in the SD-JWT" right there at https://datatracker.ietf.org/doc/html/draft-ietf-oauth-selective-disclosure-jwt-05#appendix-A.4 which is, I think, part of why Orie filed #320 asking that the example content reflect that statement by having a cnf.

[OR13]

Also for context, we are trying to use whatever examples you folks define in the official WG test suite:

https://github.com/w3c/vc-jose-cose-test-suite/blob/main/testcases/secured-vc-jwt-sd/spec.yaml