Added support for EC keys

[deltadecay]

Hello,
I've added support for EC key pairs and method to generate one from builtin curve id or name.

Example:

using OpenSSL
#curves = ec_builtin_curves()

eckey = EvpPKey(ec_generate_key("secp256k1"))

[codecov]

Codecov Report

❌ Patch coverage is 77.55102% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.09%. Comparing base (10f7b7b) to head (f8e1154).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenSSL.jl	77.55%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
- Coverage   77.56%   74.09%   -3.48%     
==========================================
  Files           2        2              
  Lines        1083     1100      +17     
==========================================
- Hits          840      815      -25     
- Misses        243      285      +42     

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[aviks]

So this fails for our integration tests with OpenSSL v1.x -- can you address those somehow?