feat: Support transient identities and traits

[khvn26]

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Changes

Closes #4279.
Contributes to #4278.

This adds support for

• Top-level transient parameter
• transient attribute for traits

for the /api/v1/identities SDK endpoint (GET, POST).

When including top-level transient: true, identity is evaluated against segments but is not persisted at all.
When calling the endpoint with traits marked as transient, these traits get evaluated against segments but do not get persisted.

Additionally, Identity.update_traits does not perform two SELECT queries anymore.

How did you test this code?

Added unit tests for views and serializers.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.78%. Comparing base (e7252cb) to head (ff66486).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4325      +/-   ##
==========================================
+ Coverage   96.61%   96.78%   +0.17%     
==========================================
  Files        1196     1159      -37     
  Lines       39123    38152     -971     
==========================================
- Hits        37797    36925     -872     
+ Misses       1326     1227      -99     

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[matthewelwell]

I've added a few fairly minor comments, but one additional thing that (I think) is missing, is a test that verifies that transient traits are correctly used when evaluating segments / segment overrides. Maybe this is implicit from other tests but I certainly expected to see an explicit test for this case.

[github-actions]

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ghcr.io/flagsmith/flagsmith-frontend:pr-4325	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4325	Finished ✅	Results ✅
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