feat: Support blank identifiers, assume transient

[khvn26]

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Changes

Closes #4448.

This PR adds the following for POST /api/v1/identities:

• Allow users to provide null/blank identifiers. The identity is assigned a UUID identifier to evaluate against and is not persisted in this case.
• If an identity is provided with "transient": true, all provided traits are automatically considered transient.
• If an identifier is provided, the existing persisted traits and overrides are loaded for evaluation. In case of collisions, transient traits take precedence during evaluation but are not persisted.
• Identifiers are returned to keep consistency with Edge API.

How did you test this code?

Updated integration and unit tests with new cases.

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-4449	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-4449	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4449	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4449	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4449	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4449	Finished ✅	Results ✅

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.89%. Comparing base (e2853d7) to head (2f3af09).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4449      +/-   ##
==========================================
+ Coverage   96.85%   96.89%   +0.04%     
==========================================
  Files        1170     1174       +4     
  Lines       38812    39161     +349     
==========================================
+ Hits        37592    37947     +355     
+ Misses       1220     1214       -6     

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[matthewelwell]

I've added some comments to some specific areas in the code, but I also had one more general comment regarding the PR description.

Identifiers are returned to keep consistency with Edge API.

I'm not quite clear on this? I think perhaps it's just related to including the identifier in the API response, as we do in the Edge API.

[khvn26]

Note: changed transient identifier generation to a hash of traits as advised by Gagan's review of the Edge API PR.