Cleanup EVP_PKEY serialization

[justsmth]

Description of changes:

• Remove duplicated logic around usage of EVP_marshal_private_key, EVP_parse_private_key, EVP_marshal_public_key and EVP_parse_public_key.
• Change all usage of EC_GROUP* to const.
• Consolidate EC SEC1 and RFC 5915 parsing/marshaling into ec:encoding module.

Call-outs:

N/A

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 75.85227% with 85 lines in your changes missing coverage. Please review.

Project coverage is 92.97%. Comparing base (c358484) to head (e484aef).
Report is 158 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ec/encoding.rs	80.32%	14 Missing and 22 partials ⚠️
aws-lc-rs/src/agreement.rs	56.00%	4 Missing and 7 partials ⚠️
aws-lc-rs/src/ec/key_pair.rs	33.33%	1 Missing and 7 partials ⚠️
aws-lc-rs/src/ed25519.rs	27.27%	0 Missing and 8 partials ⚠️
aws-lc-rs/src/evp_pkey.rs	92.42%	5 Missing ⚠️
aws-lc-rs/src/ec/signature.rs	33.33%	0 Missing and 4 partials ⚠️
aws-lc-rs/src/rsa/key.rs	60.00%	0 Missing and 4 partials ⚠️
aws-lc-rs/src/ec.rs	70.00%	3 Missing ⚠️
aws-lc-rs/src/rsa/encoding.rs	50.00%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/rsa/encryption.rs	77.77%	0 Missing and 2 partials ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #669      +/-   ##
==========================================
- Coverage   95.80%   92.97%   -2.83%     
==========================================
  Files          61       69       +8     
  Lines        8143     9593    +1450     
  Branches        0     9593    +9593     
==========================================
+ Hits         7801     8919    +1118     
- Misses        342      401      +59     
- Partials        0      273     +273     

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