Adjust Verifiable Presentation endpoint + client

[dominic22]

In this ticket #677 we successfully shown that we can sign a verifiable credential with existing node js signatures. To avoid a dependency to the identity.rs-wasm in the client we will create the VP by the backend. The client can then sign this VP to have it verifiable.

• add endpoint POST /verification/create-vp <- return unsgined VP
  • receives as body: List of VerifiableCredentials or single VerifiableCredential
• add tests
• add swagger docs
• add signVerifiablePresentation as method to the client to sign the credential