Refactor rnp_op_verify_st to use C++ types instead of raw pointers.

[ni4]

Fixes #2128 as well.

[codecov]

Codecov Report

Patch coverage: 96.61% and project coverage change: +0.02% 🎉

Comparison is base (22f5b19) 83.89% compared to head (6eab494) 83.91%.
Report is 1 commits behind head on main.

❗ Current head 6eab494 differs from pull request most recent head 08a6330. Consider uploading reports for the commit 08a6330 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2129      +/-   ##
==========================================
+ Coverage   83.89%   83.91%   +0.02%     
==========================================
  Files         161      161              
  Lines       32249    32227      -22     
==========================================
- Hits        27054    27042      -12     
+ Misses       5195     5185      -10     

Files Changed	Coverage Δ
src/lib/rnp.cpp	80.26% <96.49%> (+0.11%)	⬆️
src/lib/ffi-priv-types.h	100.00% <100.00%> (ø)

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[ni4]

@antonsviridenko ping for review.