[#4395] Add flag for updating an object to the ObjectsAPI registration

[vaszig]

Closes #4395

• Added update_existing_object flag to the ObjectsAPI (both versions). This will be used in combination with submission.initial_data_reference in order to update an existing object and not recreate it.

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 (9eafad5) to head (084cf40).
Report is 516 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4538      +/-   ##
==========================================
- Coverage   96.60%   96.58%   -0.02%     
==========================================
  Files         725      725              
  Lines       24125    24146      +21     
  Branches     2870     3175     +305     
==========================================
+ Hits        23305    23321      +16     
- Misses        559      567       +8     
+ Partials      261      258       -3     

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