Concept and naming of credential_supported / credential / credential_instance

[paulbastian]

          I think this is a great improvement to the spec.

A few questions remain for me:

• Could we use this to solve the questions whether the Wallet needs to call Credential Endpoint or Batch Credential Endpoint?
• [...]

I also have the feeling that the terminology is not right here yet. I see three levels to distinguish:

1. different credential types, each corresponds to one credentials_supported section in the Issuer metadata, e.g. birth certificate and diploma
2. different credentials (here called instances in this PR), each the same credentials_supported but with different claims data, e.g. my bachelor degree and my master degree
3. different instances of the same credential (this I would call instances instead), these are only used for unlinkability purposes

We don't need to solve everything in this PR however, so I' fine if we know how to proceed with these questions in the future

Originally posted by @paulbastian in https://github.com/openid/OpenID4VCI/pull/65#pullrequestreview-1699358644