Add signature's dictionary entries on API (/ContactInfo, /Prop_AuthTime and /Prop_AuthType)

[eduperottoni]

Description of the changes

This PR adds some options that high level API doesn't support regarding to signature's dictionary entries. More specifically, adds support to coverage of the following signature entries, described in ISO-32000: /ContactInfo, /Prop_AuthTime and /Prop_AuthType.

Related to #307.

[MatthiasValvekens]

Good work! A few minor issues, but they should be all easy to address (and I'm open to discussion on some of them).

I'll also go ahead and let the CI work through your changes.

[MatthiasValvekens]

@eduperottoni Please also make the linter happy with isort --profile black --line-length 80 pyhanko pyhanko_tests and black -q -S --line-length 80 pyhanko pyhanko_tests

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fc3d7d2) 98.76% compared to head (f258829) 98.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #314   +/-   ##
=======================================
  Coverage   98.76%   98.76%           
=======================================
  Files         104      104           
  Lines       14712    14730   +18     
=======================================
+ Hits        14530    14548   +18     
  Misses        182      182           

Flag	Coverage Δ
unittests	98.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pyhanko/cli/commands/signing/__init__.py	98.79% <100.00%> (+0.01%)	⬆️
pyhanko/sign/fields.py	99.37% <100.00%> (+<0.01%)	⬆️
pyhanko/sign/signers/pdf_byterange.py	100.00% <100.00%> (ø)
pyhanko/sign/signers/pdf_signer.py	99.84% <100.00%> (+<0.01%)	⬆️

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[MatthiasValvekens]

LGTM

[MatthiasValvekens]

Rebased & merged; thanks for your contribution!

[eduperottoni]

Thank you for your review and for all the job on pyHanko, @MatthiasValvekens!