[FIPS 2024 - CHERRY_PICK] Offer P521 for signature_algorithms in client Hello (#2572)

[justsmth]

Cherry-pick onto fips-2024-09-27. Original PR for main:

• Offer P521 for signature_algorithms in client Hello #2572

I needed to resolve a merge conflict for the ssl_test.cc changes.

---

Issues:

n/a

Description of changes:

This change adds ECDSA P521 to our list of supported certificate verification algorithms in client Hello.

Call-outs:

n/a

Testing:

CI for automated tests. Manual test to confirm signature_algorithms extension contents before/after:

./build/tool/bssl s_client -connect kms.us-east-1.amazonaws.com:443

Client Hello before:

Client Hello after:

---

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.

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

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.58%. Comparing base (f1255f8) to head (2142269).

Files with missing lines	Patch %	Lines
ssl/ssl_test.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           fips-2024-09-27    #2731   +/-   ##
================================================
  Coverage            78.57%   78.58%           
================================================
  Files                  585      585           
  Lines               101047   101047           
  Branches             14311    14312    +1     
================================================
+ Hits                 79400    79407    +7     
+ Misses               21009    21003    -6     
+ Partials               638      637    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.