add registration api endpoint

[midigofrank]

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Related issue

Fixes #

Review checklist

• [ ] I have performed a self-review of my code
• [ ] I have verified that all appropriate authorization policies (:owner, :admin, :editor, :viewer) have been implemented and tested
• [ ] If needed, I have updated the changelog
• [ ] Product has QA'd this feature

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.07%. Comparing base (ae29143) to head (f479eac). Report is 3 commits behind head on main.

:exclamation: Current head f479eac differs from pull request most recent head 2d5c84c

Please upload reports for the commit 2d5c84c to get more accurate results.

Files	Patch %	Lines
...ing_web/controllers/api/registration_controller.ex	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2145      +/-   ##
==========================================
- Coverage   90.09%   90.07%   -0.03%     
==========================================
  Files         268      269       +1     
  Lines        8951     8953       +2     
==========================================
  Hits         8064     8064              
- Misses        887      889       +2     

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