Refactor key generation classes

[ni4]

This PR refactors key generation classes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.06564% with 67 lines in your changes missing coverage. Please review.

Project coverage is 84.81%. Comparing base (135775a) to head (b5b6de2). Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib/keygen.cpp	81.63%	36 Missing :warning:
src/lib/rnp.cpp	87.34%	21 Missing :warning:
src/lib/key_material.cpp	84.00%	8 Missing :warning:
src/lib/json-utils.cpp	97.72%	1 Missing :warning:
src/lib/pgp-key.cpp	90.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2284      +/-   ##
==========================================
+ Coverage   84.67%   84.81%   +0.14%     
==========================================
  Files         116      116              
  Lines       23436    23292     -144     
==========================================
- Hits        19844    19755      -89     
+ Misses       3592     3537      -55     

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[ni4]

@ronaldtse @desvxx @maxirmx ping for review!

[maxirmx]

This one is significant I will find a couple of hours to understand itearly next week

[ni4]

@maxirmx Thanks! Wanna make another also large PR, based on this one.

[ni4]

@maxirmx thanks for the review. Having static variable would have issues with multi-threaded applications, which now could use at least separate rnp_ffi_t objects in different threads.

[ni4]

Merging with two approvals and green CI. Thanks!