Update nethsm dependency to 0.5.0

This update pulls in multiple relevant changes, mainly:
- The auto-generated client is only loaded if required.  This improves
  execution times significantly, especially for simple commands.
- Key certificates are now always stored as raw bytes, not as strings,
  so the get-certificate and set-certificate commands had to be updated.
- Changing the backup and unlock passphrases now requires passing the
  current passphrase.
- Using enums instead of strings.
- Using stronger types and dataclasses for some methods.
- NetHSM API changes that are not visible in the public Python API.

On my machine, this reduces the execution time for simple commands like
version or --help from 0.9 s to 0.3 s.