refactor!: credentialSchema does not work for CTypes

[rflechtner]

fixes KILTProtocol/ticket#2747

We finally have a proper draft spec for JSON-schema descriptions of VCs - but this is not particularly compatible with our CTypes, and the latest CType version we released is even less likely to work with this.

So I wanted to explore what it would look like if we used this field for something else - namely for describing what a KiltCredentialV1 type VC has to look like. CType verification would then have to be moved elsewhere; I think it's best here to just do it as part of credential verification.

Done so far:

• Update VC exporter to output a credentialSchema where the type is JsonSchema2023 and the id references the our credential schema for KiltCredentialV1 type credentials.
• Require this credentialSchema on verification.
• Perform CType validation on proof verification instead, with automatic fetching of the respective CType definition enabled by default (can be deactivated)
• Add option to verification methods to specify the expected CType(s), which in combination with disabling the CType loader enforces that credentials follow these CType(s).
• Define a new field on which the CType of a Kilt VC is listed and easily accessible (e.g. credential type field?).

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[ntn-x2]

Your comments make sense.

[rflechtner]

@ntn-x2 I did have a concern that CredentialSchema.validateSubject may be outdated as an accessor to a function verifying a credential's subject against it's (C)type due to the proposed change. This is because this check now has nothing to do with the VC's credentialSchema property. What's your opinion on this? Should we move this to KiltCredentialV1? Or do you have another suggestion for where this functionality should live?

[ntn-x2]

What's your opinion on this? Should we move this to KiltCredentialV1?

It would make sense, right? This is pretty much tied to what we do in this version of the spec.