Add the ability to Retrieve Credential Definitions

[davideroffo]

Summary

The current implementation of the Credential Issuance tab lacks the ability to retrieve the Credential Definition. This feature would be beneficial for Agents who want to Request a Presentation for a Credential Definition that they do not own.

Consider the following scenario:

• Issuer 1 creates a Credential Schema
• Issuer 1 creates a Credential Definition for the schema
• Issuer 1 issues a Credential to Holder 1
• Verifier 1 wants to Request a Presentation from Holder 1 for the Credential that Issuer 1 issued
• Aries Toolbox does not allow Verifier 1 to do this if Verifier 1 does not own the Credential Definition

To address this issue, we should add a "Credential Definitions retrieve" feature, similar to the "Schemas retrieve" feature.