Extendable Issuer Property for VC issuance [OpenBadges 3.0]

[bsandmann]

Summary

This feature request is to modify the Hyperledger Identus agent to allow for a flexible issuer property in Verifiable Credentials (VCs). This change is necessary to achieve full compatibility with OpenBadges 3.0 and to provide more versatility in describing the issuer of a credential.
For further discussion on what OpenBadges 3.0 is and why to achieve compatibility with it, please refer to the Achieve compatibility with OpenBadges 3.0 [Summary].

Current Situation

Currently, the Identus agent uses a string value for the issuer property when creating a VC:

"issuer": "did:prism:123..."

Proposed Changes

To align with the OpenBadges 3.0 specification, which allows for a more detailed issuer description, and to enhance the versatility of Identus-issued credentials, I propose to make the issuer property optionally accept an object within the current API to replace the issuer-string.
The structure of that optional property might look like this in the existing endpoint for offering credentials: /issue-credentials/credentials-offers:

"issuer": {
"name": "Example University",
"type": ["Profile"],
"image": "https://example.edu/logo.png"
}

• The provided json is not allowed to contain the id field, which should be set by the Identus agent based on the DID of the issuer.
• If the issuer property is not set the current behavior should be maintained (ie. "issuer":"did:prism:123").

This change also requires changes in the SDKs to handle the issuer object correctly, since they are currently expecting a string with the DID.

Additional resources:

Issuer explanation from the VC 2.0 spec
Example of Issuer object from the OB 3.0 spec