Add API endpoint for default fixed metadata variables of JSON dump plugin

[viktorvanwijk]

Closes #5087

Changes

Add API endpoint for default fixed metadata variables of JSON dump plugin.

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

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

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

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.74%. Comparing base (47c74d6) to head (1ab81fd).

Files with missing lines	Patch %	Lines
...forms/registrations/contrib/json_dump/api/views.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5094      +/-   ##
==========================================
- Coverage   96.74%   96.74%   -0.01%     
==========================================
  Files         771      773       +2     
  Lines       26636    26646      +10     
  Branches     3467     3467              
==========================================
+ Hits        25770    25778       +8     
- Misses        605      607       +2     
  Partials      261      261              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.