feat: Update signer-function logic to be more dynamic

[cre8]

Right now I can pass a signer function to request the required key on demand like this example with veramo:

    const signer: Signer = async (data: string) => context.agent.keyManagerSign({ keyRef: key.kid, data })
    const sdjwt = new SDJwtInstance({
      signer,
      hasher: this.algorithms.hasher,
      saltGenerator: this.algorithms.salltGenerator,
      signAlg: alg,
  })

The problem is, that the signAlg is a static value so I am not able to use any other sign algorithm in my signer.

Suggestion

The signer will return not just the signature, but also the used signAlgorithm. This would allow a more dynamic approach We could think about using this approach also for the hasher, beside the specification of sd-jwt is limited to sha-256. But It would make this approach more generalised when we change it now for the signature