Define terms for cryptographic and non-dataset credential data, and update Batch Credential Issuance dimensions

The Batch Credential Issuance section currently describes Credentials as varying along three dimensions:

• Credential Format
• Credential Dataset
• Cryptographic Data

However, “Cryptographic Data” is used as a concept but does not appear to have its own terminology definition. We should add a definition for it.

Recent discussions around Credential Dataset Version also clarifies that Credential instances with the same Credential Dataset may differ not only in cryptographic data, such as Issuer signatures or key binding material, but also in other data that is not part of the Credential Dataset, such as timestamps. See https://github.com/openid/OpenID4VCI/pull/472#discussion_r3402086680.

We should introduce terminology for this broader category as well, for example "Credential instance-specific data" or a better term, and use it consistently. This term should cover data that may differ between Credential instances without changing the Credential Dataset.

The Batch Credential Issuance section should then be updated so the listed dimensions are not limited to Credential Format, Credential Dataset, and Cryptographic Data only. It should include the new broader term, with Cryptographic Data described as one example of that category.

We should also define Cryptographic Data as a term.

Start with the Batch Credential Issuance section and the Credential Dataset Version discussion linked in the issue. Determine the broader term for data that can vary between Credential instances without changing the Credential Dataset, define Cryptographic Data, and update the dimensions list so the relationship is consistent. Done means both terms are defined and the section uses the broader category correctly.