Logo in Credential Issuer should be a JSON object

I'm currently working on conformance tests for OID4VCI in the context of the OIDF Conformance Testing suite and I'm using
this Keycloak branch as one of the implementations to build the tests with.

The conformance tests will be based on the ID2 version 15. In the latest spec version, the value of the logo property must be a JSON object.
See: https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0-ID2.html#section-11.2.3-2.10.2.3.1

However in the current implement it is rendered as a string:

This currently triggers some validation issues:
```
[
{
"error": "string found, object expected",
"path": "credential_configurations_supported/SteuerberaterCredential/display[0]/logo"
},
{
"error": "string found, object expected",
"path": "credential_configurations_supported/SteuerberaterCredential/display[1]/logo"
}
]
```