Support for sign/verify w/ digest

[justsmth]

Description of changes:

• Support for signing/verifying precomputed digests.
  • Supported by RSA and ECDSA.
  • ML-DSA and Ed25519 do not allow precomputed digests to be used.
• Added/updated documentation.

Testing:

• Update existing tests to also cover precomputed digests.

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 92.03540% with 18 lines in your changes missing coverage. Please review.

Project coverage is 92.39%. Comparing base (c358484) to head (2819d6e).
Report is 234 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/evp_pkey.rs	88.57%	4 Missing and 4 partials ⚠️
aws-lc-rs/src/rsa/signature.rs	92.30%	1 Missing and 2 partials ⚠️
aws-lc-rs/src/ec/key_pair.rs	85.71%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/ec/signature.rs	93.93%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/rsa/key.rs	91.66%	1 Missing and 1 partial ⚠️
aws-lc-rs/src/digest.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
- Coverage   95.80%   92.39%   -3.41%     
==========================================
  Files          61       71      +10     
  Lines        8143     9280    +1137     
  Branches        0     9280    +9280     
==========================================
+ Hits         7801     8574     +773     
- Misses        342      430      +88     
- Partials        0      276     +276     

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