Prevent using unrestricted proof-requests OR handle self-attested attributes properly

[esune]

As discovered by investigation for #108, it is not possible to use unrestricted proof requests with vc-authn: the proof request is generated correctly, but then the controller throws an error when trying to fetch the claim values for the proof.

The options are:

• do NOT allow unrestricted proof requests to be configured/used in vc-authn: this is acceptable, based on the use cases that vc-authn is usually used for, as self-attested attributes are somewhat bound to limited/insecure scenarios. In this case, update the controller code to reject unrestricted proof configurations and update the documentation to reflect the change
• handle correctly also proof-requests containing unrestricted claims: the agent (aca-py) supports this so it would be consistent with the capabilities it provides