Add custom signing function for the adobeX509RSASHA1 signature handler

[adrg]

Allow specifying a custom signing function for the adobeX509RSASHA1 signature handler

[codecov]

Codecov Report

Merging #414 into v3 will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##               v3     #414      +/-   ##
==========================================
- Coverage   54.38%   54.37%   -0.02%     
==========================================
  Files         153      153              
  Lines       27671    27680       +9     
==========================================
  Hits        15050    15050              
- Misses      10825    10834       +9     
  Partials     1796     1796

Impacted Files	Coverage Δ
pdf/model/sighandler/sighandler_rsa_sha1.go	4.81% <0%> (-0.59%)	⬇️

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