digestsign interface for EVP_DigestSign cryptographic signatures

[imyxh]

Add initial support for the OpenSSL EVP_DigestSign interface, as per #49. At least RSA signing should work now. Also tidy up some stuff (closes #28 on the side).

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (10f7b7b) to head (3c04503).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenSSL.jl	86.66%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   77.56%   74.69%   -2.87%     
==========================================
  Files           2        2              
  Lines        1083     1075       -8     
==========================================
- Hits          840      803      -37     
- Misses        243      272      +29     

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[imyxh]

I don't know what's going on with the nightly tests but I don't think they're my fault. As for lts, do we want to support OpenSSL v1 still? EVP_DigestSignUpdate() was converted from a macro to a function in OpenSSL 3.0. I can try to support pre-v3 by switching on the openssl version and just calling DigestUpdate instead, but personally I find this a bit cursed. OpenSSL v3 is stable in virtually every environment known to man.

[aviks]

I can try to support pre-v3 by switching on the openssl version

I don't think we have consensus on dropping v1, so maybe you can throw some kind of not-implemented exception at the very least?