Signature refactoring

[adrg]

• Remove duplicate code in Signature model ToPdfObject
• Update signature object ByteRange field in the appender write method

[codecov]

Codecov Report

Merging #358 into v3 will decrease coverage by 5.39%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##               v3    #358     +/-   ##
========================================
- Coverage   60.59%   55.2%   -5.4%     
========================================
  Files         152     152             
  Lines       27304   27301      -3     
========================================
- Hits        16546   15072   -1474     
- Misses      10364   10375     +11     
- Partials      394    1854   +1460

Impacted Files	Coverage Δ
pdf/model/signature.go	77.5% <100%> (-4.47%)	⬇️
pdf/model/appender.go	75.99% <100%> (-6.89%)	⬇️
pdf/creator/utils.go	55.55% <0%> (-22.23%)	⬇️
pdf/core/security/handlers.go	33.33% <0%> (-16.67%)	⬇️
pdf/model/optimize/image_ppi.go	56.71% <0%> (-16.42%)	⬇️
pdf/model/reader.go	50.5% <0%> (-15.94%)	⬇️
pdf/ps/exec.go	62.96% <0%> (-14.82%)	⬇️
pdf/ps/operations.go	53.45% <0%> (-13.84%)	⬇️
pdf/model/signature_handler.go	55.04% <0%> (-13.77%)	⬇️
pdf/extractor/image.go	66.97% <0%> (-13.77%)	⬇️
... and 96 more

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