[#5957] add payment related fields to v3 form endpoint

[SonnyBA]

Closes #5957

[skip: e2e]

Changes

Adds payment related fields to the v3 form endpoint.

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
  • Checked new model fields are usable in the admin
  • Problem detection in the admin email digest is handled

• 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

• Documentation

  • Added documentation which describes the changes

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.07%. Comparing base (4142dff) to head (96ed4a8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/openforms/forms/api/v3/serializers/payment.py	94.11%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6033      +/-   ##
==========================================
- Coverage   97.07%   97.07%   -0.01%     
==========================================
  Files         883      884       +1     
  Lines       33225    33265      +40     
  Branches     2978     2979       +1     
==========================================
+ Hits        32254    32292      +38     
- Misses        655      656       +1     
- Partials      316      317       +1     

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