Refactor (accessing) the cosign state

[sergei-maertens]

Closes #4320

Changes

This refactor sets up a dedicated interface to read the cosign state of a submission. Highlights:

• Good typing support
• Ability to deal with cosign v1 and v2 details in the proper context
• Proper factory trait to set up the necessary database state of a submission
• Prepared for cosign v3

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.61%. Comparing base (2084974) to head (efa8791).
Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4888      +/-   ##
==========================================
+ Coverage   96.60%   96.61%   +0.01%     
==========================================
  Files         752      753       +1     
  Lines       25812    25879      +67     
  Branches     3412     3416       +4     
==========================================
+ Hits        24935    25003      +68     
  Misses        613      613              
+ Partials      264      263       -1     

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