Support PKCS8 in agreement::PrivateKey::from_private_key_der

[justsmth]

Description of changes:

• Recent refactoring dropped support for parsing PKCS8 from agreement::PrivateKey::from_private_key_der. This PR restores that support.
• Bump version to 1.12.5

Call-outs:

• We had no function to serialize an ECDH private key to PKCS8. In order to test this, this functionality needed to be added.

Testing:

• Updated test.

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 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.86%. Comparing base (c358484) to head (4e7836f).
Report is 177 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/agreement.rs	83.33%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #713      +/-   ##
==========================================
- Coverage   95.80%   92.86%   -2.94%     
==========================================
  Files          61       70       +9     
  Lines        8143     9561    +1418     
  Branches        0     9561    +9561     
==========================================
+ Hits         7801     8879    +1078     
- Misses        342      413      +71     
- Partials        0      269     +269     

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