Fix: id field in SignUpResponse

[tanya732]

This PR addresses inconsistency in the SignUpResponse object by ensuring the id field is always available, regardless of which identifier field the Auth0 API returns i.e., _id, user_id or id

fixes #991

This inconsistency caused issues for developers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.20%. Comparing base (3fda381) to head (d1fd7ed).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1190    +/-   ##
========================================
  Coverage   79.19%   79.20%            
========================================
  Files         580      580            
  Lines       18150    18155     +5     
  Branches     8480     8617   +137     
========================================
+ Hits        14374    14379     +5     
  Misses       3776     3776            

Flag	Coverage Δ
alltests	79.20% <100.00%> (+<0.01%)	⬆️

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