Make Issuer metadata normative

[paulbastian]

As I stated in the WG Call, I believe that credential_configuration_id is the better choice. In general, I have trouble understanding how OpenID4VCI works well without metadata, as the Wallet needs to known:

• the credential endpoint
• supported proof types
• supported credentials and formats
• display data for the issuer and the credential

Therefore I believe in productive environment it will be very common that the Issuer has the ability to host metadata.

Originally posted by @paulbastian in https://github.com/openid/OpenID4VCI/pull/219#pullrequestreview-1833193401

As stated in #219 I believe that Credential Issuer metadata should be mandatory, it just doesn't make sense to me with the current specification otherwise.