Test PKCS#11 backend with a softhsm2 token

softhsm2 is a soft token, which is available in many distributions like Fedora and Debian.

Add a CI job that runs the test suite against a softhsm2 token.

Note: softhsm2 does not include PQC support, and some functionality is missing. Support for softhsm2 is on a best-effort basis to improve our implementations interoperability by making it harder to overfit to the project's primary target, kryoptic.