Add weekly automated check for outdated third-party test vectors

[sgmenda]

Issues:

We currently rely on manual updates of test vectors from third-party sources like Wycheproof. This PR starts the process of automating this workflow.

Description of changes:

Adds a GitHub Actions workflow that runs every Wednesday at 10 AM PT and opens a PR with update instructions if any of the vectors we use need updating.

Call-outs:

• This only works for vectors onboarded to the new third_party/vectors system introduced in Add infrastructure for managing third-party test vectors #2811
• This workflow only notifies that vectors need updating and does not automatically update them
• It only checks vectors we actually use, not all available Wycheproof vectors
• It opens at most one PR per week and assigns it to me

Testing:

The key component of this workflow is the sync.py --check command, which I tested locally and confirmed works correctly. The workflow can be manually triggered to debug any issues.

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 78.21%. Comparing base (8238483) to head (931b73b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2933      +/-   ##
==========================================
- Coverage   78.22%   78.21%   -0.01%     
==========================================
  Files         690      690              
  Lines      118745   118745              
  Branches    16680    16679       -1     
==========================================
- Hits        92884    92882       -2     
  Misses      24972    24972              
- Partials      889      891       +2     

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