Enable disabling of Signature Verification by configuration

Not just in the /verification frontend (see demo here), but also in the overall operation of the Catalogue (e.g., via the API), it should be configurable whether or not the verification of signatures should be enforced. Verification of signatures does not currently support the state of the art of how Self-Descriptions are signed with recent implementation of Gaia-X (see #178 (closed)).

For configurability, we need a general feature, as we also need this for #173. @moosmannp could you please create a new issue for that?