Add EC_GROUP mutablility to custom curves

[samuel40791765]

Issues:

Resolves CryptoAlg-2547

Description of changes:

Following up on #1860 which introduces an API to return a mutable EC_GROUP, this commit applies the same concept for custom curves. Custom curves are already dynamically allocated, but were logically immutable due to the strict enforcement of calling EC_GROUP_new_curve_GFp -> EC_GROUP_set_generator.
The reference call in EC_GROUP that was used for custom curves is no longer needed. We actually duplicate the structure instead.

Call-outs:

N/A

Testing:

Test using a custom generator identical to P-256.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.51%. Comparing base (ca8180c) to head (bf170e6).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1881      +/-   ##
==========================================
+ Coverage   78.49%   78.51%   +0.02%     
==========================================
  Files         585      585              
  Lines       99630    99712      +82     
  Branches    14254    14262       +8     
==========================================
+ Hits        78206    78291      +85     
+ Misses      20789    20786       -3     
  Partials      635      635              

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