Allow user registration via API

[midigofrank]

Validation Steps

curl -X POST \
  -H "Content-Type: application/json" \
  -d '{"user": {"first_name": "test", "last_name": "last", "email": "testemail@openfn.org", "password": "mypassword", "terms_accepted": true}}' \
  http://localhost:4000/api/users/register

Notes for the reviewer

• Uses the same controller action for user registration.

Related issue

Fixes OpenFn/thunderbolt#125

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (50e2efd) to head (7e7c794).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    OpenFn/lightning#2043      +/-   ##
==========================================
+ Coverage   90.04%   90.06%   +0.02%     
==========================================
  Files         249      249              
  Lines        8219     8226       +7     
==========================================
+ Hits         7401     7409       +8     
+ Misses        818      817       -1     

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[midigofrank]

Issue moved to thunderbolt