Fix certutil certificate listing

[sosthene-nitrokey]

This PR contains 2 changes:

• Add the CKA_SERIAL_NUMBER attribute to certificate objects.
• Fix fetch_all_keys request caching a subset of all objects, using only the kind that was first requested.

This doesn't fix the certificate upload part of #183, but that cannot be fixed without support for certificates without associated private keys.

This does fix certificate listing with certutil mentionned in #187.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (13fa492) 0.00% compared to head (1defba0) 90.36%.

Files	Patch %	Lines
pkcs11/src/backend/session.rs	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #185 +/- ## ========================================= + Coverage 0 90.36% +90.36% ========================================= Files 0 31 +31 Lines 0 6642 +6642 ========================================= + Hits 0 6002 +6002 - Misses 0 640 +640 ```

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[rcritten]

I tested this patch and confirmed that it can list certificates uploaded using a different mechanism, and that certutil cannot add certificates.