Parse signature certificate arrays on signature validation

[adrg]

Addresses #174

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This change is 

[codecov]

Codecov Report

Merging #175 into development will decrease coverage by 0.02%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development     #175      +/-   ##
===============================================
- Coverage        57.55%   57.52%   -0.03%     
===============================================
  Files              189      189              
  Lines            34413    34430      +17     
===============================================
  Hits             19806    19806              
- Misses           12059    12076      +17     
  Partials          2548     2548

Impacted Files	Coverage Δ
model/sighandler/sighandler_pkcs7.go	52.87% <0%> (-8.46%)	⬇️
model/sighandler/sighandler_rsa_sha1.go	4.54% <0%> (-0.28%)	⬇️

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