Convert multibase key to a JWK key for DID verification methods (e.g. did:cheqd)

[philpotisk]

Is your feature request related to a problem? Please describe.

In class DidCheqdResolver the key in the verification method should be converted from Multibase to JWK format in order to fix did:cheqd resolution and related issues https://github.com/walt-id/waltid-identity/issues/656

Describe the solution you'd like We need to introduce the general key-convertion functionality to be used in DidCheqdResolver or else where. Probably the code from waltid-ssikit can be reused.