Signature appearance

[adrg]

Add text based signature appearance.
Addresses #340 and #323 (except for the signature appearance image support which will be addressed in another PR)

[codecov]

Codecov Report

Merging #354 into v3 will increase coverage by 0.39%.
The diff coverage is 79.05%.

@@            Coverage Diff             @@
##               v3     #354      +/-   ##
==========================================
+ Coverage   54.82%   55.21%   +0.39%     
==========================================
  Files         152      152              
  Lines       27161    27294     +133     
==========================================
+ Hits        14890    15070     +180     
+ Misses      10432    10371      -61     
- Partials     1839     1853      +14

Impacted Files	Coverage Δ
pdf/model/fields.go	49.71% <100%> (+0.87%)	⬆️
pdf/model/appender.go	76.13% <60%> (-0.36%)	⬇️
pdf/annotator/field_appearance.go	10.87% <77.27%> (+10.87%)	⬆️
pdf/annotator/form_field.go	13.57% <82.6%> (+13.57%)	⬆️
pdf/core/primitives.go	75.26% <0%> (-0.64%)	⬇️
pdf/model/colorspace.go	32.86% <0%> (+0.13%)	⬆️
pdf/contentstream/creator.go	46.48% <0%> (+8.71%)	⬆️
pdf/model/xobject.go	58.01% <0%> (+11.37%)	⬆️

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[gunnsth-review]

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