[#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

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

• Release management

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

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

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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