Digital signatures refactoring

[adrg]

Improve PDF sign method signature.

[codecov]

Codecov Report

Merging #350 into v3-a5i-pdf-sign will increase coverage by 0.1%.
The diff coverage is 90.19%.

@@                Coverage Diff                 @@
##           v3-a5i-pdf-sign     #350     +/-   ##
==================================================
+ Coverage            58.44%   58.55%   +0.1%     
==================================================
  Files                  149      148      -1     
  Lines                25716    25658     -58     
==================================================
- Hits                 15031    15024      -7     
+ Misses               10313    10265     -48     
+ Partials               372      369      -3

Impacted Files	Coverage Δ
pdf/model/fields.go	54.94% <100%> (+1.54%)	⬆️
pdf/model/appender.go	83.12% <78.94%> (-0.29%)	⬇️
pdf/model/signature.go	83.33% <95%> (+26.47%)	⬆️
pdf/internal/sampling/resample.go	94.11% <0%> (-0.85%)	⬇️
pdf/model/functions.go	48.89% <0%> (-0.23%)	⬇️
pdf/model/colorspace.go	37.65% <0%> (ø)	⬆️
pdf/model/annotations.go	23.17% <0%> (+0.08%)	⬆️
pdf/core/primitives.go	78.6% <0%> (+0.63%)	⬆️

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[gunnsth-review]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[sayhei]

can have an example of how to use digital sign?