Generating a nonce for a proof request

[berendsliedrecht]

Right now, It not really possible to generate a nonce, via proofService.generateNonceForProofRequest() without a lot of hassle because you need to instantiate the proofService.

We should add a way to generate this nonce without instantiating this whole class, or we should create it for the user if nothing is provided in the options of the constructor for the proofRequest class.