[#4762] Add cosign data (bsn) to the stuf-zds creerzaak bericht

[vaszig]

Closes #4762

Changes

• Added the cosigner's bsn to the creerzaak bericht

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 96.58%. Comparing base (796bb43) to head (f502fef).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4835   +/-   ##
=======================================
  Coverage   96.58%   96.58%           
=======================================
  Files         748      748           
  Lines       25476    25477    +1     
  Branches     3368     3368           
=======================================
+ Hits        24605    24606    +1     
  Misses        608      608           
  Partials      263      263           

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