Unstable API for ML-DSA

[justsmth]

Description of changes:

• Adds unstable API for support of MLDSA.

Call-outs:

• For the signature operation, I decided to have the caller provide the mutable buffer. The required buffer length is provided by PqdsaSigningAlgorithm::signature_len(). This is similar to RSA's signature function.
• We don't yet have a way of obtaining the seed value from the PqdsaKeyPair. So, we can marshal from a seed, but not serialize to it.
• PqdsaPrivateKey object can serialize to either PKCS8 or raw bytes.
• PqdsaPublicKey object can serialize to X.509 or raw bytes.
• PqdsaSeed object can't do anything (yet).

Testing:

New tests added.

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

Attention: Patch coverage is 91.22257% with 28 lines in your changes missing coverage. Please review.

Project coverage is 92.93%. Comparing base (c358484) to head (76dd568).
Report is 175 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/pqdsa/key_pair.rs	89.60%	6 Missing and 15 partials ⚠️
aws-lc-rs/src/pqdsa/signature.rs	93.18%	0 Missing and 3 partials ⚠️
aws-lc-rs/src/pqdsa.rs	96.61%	2 Missing ⚠️
aws-lc-rs/src/ec/encoding.rs	66.66%	0 Missing and 1 partial ⚠️
aws-lc-rs/src/test.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #690      +/-   ##
==========================================
- Coverage   95.80%   92.93%   -2.87%     
==========================================
  Files          61       71      +10     
  Lines        8143     9801    +1658     
  Branches        0     9801    +9801     
==========================================
+ Hits         7801     9109    +1308     
- Misses        342      407      +65     
- Partials        0      285     +285     

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