fix for issue 124 (Signature size for RSA 3072 and 4096 bits is too small)

[mitresthen]

Add a hopefully more correct way of calculating signature size, based on how it's done in other pdf signing libraries. Set an upper limit on retries to avoid a loop. Reset state inbetween retries to avoid corrupt pdf's.

[codecov]

Codecov Report

❌ Patch coverage is 70.73171% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.03%. Comparing base (b62ffc7) to head (c9c59a5).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
sign/keysize.go	72.72%	6 Missing and 6 partials ⚠️
sign/pdfsignature.go	46.66%	6 Missing and 2 partials ⚠️
sign/sign.go	82.60%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   60.42%   62.03%   +1.60%     
==========================================
  Files          25       26       +1     
  Lines        1908     1973      +65     
==========================================
+ Hits         1153     1224      +71     
+ Misses        590      579      -11     
- Partials      165      170       +5     

Flag	Coverage Δ
unittests	62.03% <70.73%> (+1.60%)	⬆️

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