feat: implement Verification Method relationship for Credential Request

[Oran-Dan]

Description

To comply with the DIIP v4 profile Credential Requests must include the jwt proof type. The jwt must have the did of the Unicore instance as the iss value of the jwt proof. For did:web the kid must be of the key in the assertionMethod of the did in the iss value.
For did:jwk and did:key there is only one key so the Verification Method relationship is not applicable, did:iota is out of scope for this issue,

Motivation

Comply with DIIPv4

Resources

https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0-15.html#name-proof-types
https://fidescommunity.github.io/DIIP/

To-do List

• Make sure did:web instances use the "assertionMethod" key as kid
• Item two
• Item three