Add support for Issue Credential 2.1 -- Issuer and Holder

[swcurran]

In order to support OCA For Aries (PR here, we need to support Issue Credential v2.1, so that we can then add support for Issue Credential v2.2 (currently a PR).

Task is to design and then implement adding support for the Issue Credential v2.1, noting that we will be adding support for Issue Credential 2.2 soon after. We must continue to support as the minimum minor version v2.0 as this is done. There will be impacts on the Admin API to implement this.

Please prepare a high-level design and add any questions to this issue for discussion.