`OpenId4VPConfig.issuer` is not used

[dzarras]

The property OpenId4VPConfig.issuer is not used by the library.

• When the Request Object is provided by value, or is fetched by reference using Request URI Method Get the library must verify that the aud claim of the Request Object is https://self-issued.me/v2
• When the Request Object is fetched by reference using Request URI Method Post, the library must advertise OpenId4VPConfig.issuer in the issuer claim of the Authorization Server Metadata, and the verify that the aud claim of the Request Object matches this value when Wallet Metadata are included in the request.

[dzarras]

Related issue: openid/OpenID4VP#684