Make rnp_signature_handle_st C++ object

[desvxx]

fixes #2271

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.80%. Comparing base (3bd1b71) to head (fa417d9).

Files with missing lines	Patch %	Lines
src/lib/rnp.cpp	84.61%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2279 +/- ## ========================================== - Coverage 84.81% 84.80% -0.01% ========================================== Files 116 116 Lines 23292 23280 -12 ========================================== - Hits 19755 19743 -12 Misses 3537 3537 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests