Consolidate agreement functions into LcPtr<EVP_PKEY>

[justsmth]

Description of changes:

We previously had two nearly identical methods for performing agreement. This change consolidates them into an agree function in LcPtr<EVP_PKEY>

Testing:

No changes to tests.

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 83.78378% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.35%. Comparing base (c358484) to head (5aaa88f).
⚠️ Report is 239 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/evp_pkey.rs	76.19%	4 Missing and 1 partial ⚠️
aws-lc-rs/src/agreement.rs	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #854      +/-   ##
==========================================
- Coverage   95.80%   92.35%   -3.45%     
==========================================
  Files          61       71      +10     
  Lines        8143     9277    +1134     
  Branches        0     9277    +9277     
==========================================
+ Hits         7801     8568     +767     
- Misses        342      429      +87     
- Partials        0      280     +280     

☔ 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.