Partial response encryption

As part of the discussion of https://github.com/openid/OpenID4VCI/issues/339 we discussed whether there is a need partial response encryption and to have a separate discussion item for that to give reasons for having partial response encryption.

Conceptually partial response encryption is important when there are different components involved in provisioning that don't all have access to the key used for encryption, but they need access to part of the response to provide functionality. Examples of such situations are:

• Within the mobile device there are areas on the mobile device that not all have access to the key used for response encryption, but they may need access to part of the response.
• The mobile device has secure areas outside of the mobile device that contains the key used for response encryption, but components on the mobile device need access to part of the response.
• Issuance is mediated by a wallet server, where the wallet server needs access to part of the response but doesn't have access to the key used for response encryption.

Start with the discussion in issue #339 and the examples in this issue. Determine whether partial response encryption is required and what distinct components need to access. Done means the discussion reaches a clear decision and records the resulting requirements or specification changes.