Fix regression in EcdsaKeyPair::from_private_key_der

[justsmth]

Description of changes:

• EcdsaKeyPair::from_private_key_der should accept either RFC 5208 or RFC 5915 encodings.

Call-outs:

• I found this problem while investigating the failure of our rustls integ tests.

Testing

• Added a test to prevent future regression.
• I verified that this fixes the test failure in rcgen.

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 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.89%. Comparing base (c358484) to head (c731228).
Report is 162 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ec/key_pair.rs	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
- Coverage   95.80%   92.89%   -2.91%     
==========================================
  Files          61       69       +8     
  Lines        8143     9565    +1422     
  Branches        0     9565    +9565     
==========================================
+ Hits         7801     8885    +1084     
- Misses        342      399      +57     
- Partials        0      281     +281     

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