feat: Breaking change: support conjunctions of disjunctions of conjunctions when requesting attributes in any of the three supported session types

Instead of requesting several attributes, one can now request several groups of attributes. Within each such group, attributes belonging to the same credential type always come from the same credential instance. This affects all three session types, as in each of them attributes can be disclosed.

feat: Support --url for session. Some code duplication was unavoidable without splitting up requestorserver.initialize

When disclosing two attributes from the same credential instance, irmaclient would disclose both attributes twice.

irma scheme verify now errors if an issuer's latest public key does not support sufficient attributes

Verifying an individual scheme no emits the same warnings and errors as verifying multiple schemes.