Add MLDSA to fuzzing corpus

[jakemas]

Issues:

Resolves #PQCrypto-120

Description of changes:

This PR updates the fuzz tests to include seeds based upon ML-DSA private key encodings. I provided manual seeds to the corpus to fuzz specifically against private key import.

Seed1: ML-DSA-44 private key:

 -----BEGIN PRIVATE KEY-----
 MDICAQAwCwYJYIZIAWUDBAMRBCAAAQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRob
 HB0eHw==
 -----END PRIVATE KEY-----

Seed2 ML-DSA-65 private key:

-----BEGIN PRIVATE KEY-----
MDICAQAwCwYJYIZIAWUDBAMSBCAAAQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRob
HB0eHw==
-----END PRIVATE KEY-----

Seed1: ML-DSA-87 private key:

-----BEGIN PRIVATE KEY-----
MDICAQAwCwYJYIZIAWUDBAMTBCAAAQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRob
HB0eHw==
-----END PRIVATE KEY-----

Methodology:

• fuzz new seed values: ./fuzz/pkcs8 -max_len=2048 -jobs=32 -workers=32 mldsa-seed/
• reduce down to a minimized corpus: ./fuzz/pkcs8 -merge=1 min-corpus/ mldsa-seed/
• check coverage by introducing an error (emulated Fix issue with ML-DSA key parsing #2152 and verified these fuzzes)
• merge into existing corpus: ./fuzz/pkcs8 -merge=1 merged/ min-corpus/ ../fuzz/pkcs8_corpus/
• PR this new corpus.

As an example, the new file 2aedfe1f6ee6e99c5947577cc6d59a9611f2a085 contains the fuzz clearly generated from the ML-DSA seeded BASE64 encoding:

MDICAQAwKwYJKsOcSMOcy5wKAQEKMB7CogICAMucw5p+wq7Fk35+fsOHwrDLh8uHy4dCy4YwIwJe
y53Lh8WTAAAKMQQAy4c=

Call-outs:

Wasn't able to get a coverage report from https://github.com/aws/aws-lc/blob/main/docs/Coverage.md as lcov didn't want to play with clang.

We were at 231 fuzz vectors in pkcs8_corpus and now we are at 333.

Testing:

Testing coverage was tested by introducing an error (emulated #2152). These tests caught the error.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.00%. Comparing base (f407534) to head (540548e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2174      +/-   ##
==========================================
+ Coverage   78.98%   79.00%   +0.01%     
==========================================
  Files         611      611              
  Lines      105904   105905       +1     
  Branches    14982    14982              
==========================================
+ Hits        83651    83671      +20     
+ Misses      21598    21580      -18     
+ Partials      655      654       -1     

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[andrewhop]

Verified new corpus resulted in a step up in pkcs8 block coverage.