fix: DEV-3430: Adding CreateOnlyFieldsMixin and adding to BaseUserSerializer to make email only writeable on create

[wesleylima]

The approach here was to create a mixin that allows developers to define create_only_fields in a Serializer's Meta class.
I believe this feature doesn't exist in the codebase yet and I think it's likely it will be reused.

The behavior when a "create_only" field is added on an update operation is that the requests completes successfully while ignoring any fields that were "create_only". This is analogous to how django-rest-api Serializer handles read_only fields.

[swarmia]

✅  Linked to Bug DEV-3430 · Insufficient Authorization Checks – E-Mail Field
➡️  Part of Epic DEV-3239 · Customer Issues - Q3 Sprint 4

[codecov]

Codecov Report

Base: 77.80% // Head: 77.51% // Decreases project coverage by -0.28% ⚠️

Coverage data is based on head (0ca0ae7) compared to base (17b40da).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3027      +/-   ##
===========================================
- Coverage    77.80%   77.51%   -0.29%     
===========================================
  Files          137      146       +9     
  Lines         9951    10790     +839     
===========================================
+ Hits          7742     8364     +622     
- Misses        2209     2426     +217     

Impacted Files	Coverage Δ
label_studio/label_studio/users/mixins.py	75.00% <0.00%> (-12.50%)	⬇️
...bel_studio/label_studio/organizations/functions.py	66.66% <0.00%> (-10.26%)	⬇️
label_studio/label_studio/ml/api.py	83.69% <0.00%> (-6.85%)	⬇️
...l_studio/label_studio/core/templatetags/filters.py	48.75% <0.00%> (-6.20%)	⬇️
..._studio/label_studio/core/settings/label_studio.py	94.28% <0.00%> (-5.72%)	⬇️
label_studio/label_studio/organizations/api.py	79.04% <0.00%> (-5.33%)	⬇️
.../label_studio/data_manager/actions/experimental.py	14.79% <0.00%> (-4.72%)	⬇️
label_studio/label_studio/tasks/api.py	88.26% <0.00%> (-4.43%)	⬇️
label_studio/label_studio/tasks/serializers.py	84.18% <0.00%> (-2.37%)	⬇️
label_studio/label_studio/data_manager/managers.py	79.49% <0.00%> (-2.22%)	⬇️
... and 76 more

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