Verifier app attestation for Wallet Instances

[grausof]

It is necessary to define a verification app attestation mechanism (similar to the Wallet Instance Attestation) to be presented to the Wallet Instance at the credentials request stage and also working offline.

[fmarino-ipzs]

According to ISO, this should require Verifier App Authentication and it should be done using x.509 access certificate provided by the Verifier app provider to the instance.