Proof-Request Explorer

[esune]

Publicly accessible UI to search available proof-request configurations, see #226

Some thoughts:

• it might be preferable to not require an extra service to be deployed alongside VC-AuthN, but this is not a deal breaker
• a Vue app - served by the back-end or by a separate webserver - should fit the bill
• we may need to define top-level "standard" metadata and related filters, and let the users specify further filtering using GraphQL or similar
• APIs to query results should still be protected, as we don;t want to risk the service being spammed by applications trying to fetch information

[loneil]

POC version deployed. Iterating on future changes and requirements with Kim.
See details in #514