fix: Rely on Flagsmith Engine for segment evaluation, avoid N+1 queries

[khvn26]

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Changes

1. Add the prefetch_related to SDKIdentitiesDeprecated. This solves the traits N+1 query.
2. Move the mapper invocations out of the related segment iteration loop. This solves the environments N+1 query.

How did you test this code?

[vercel]

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[github-actions]

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[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e5058ae) 95.73% compared to head (8c0b0b9) 95.88%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
api/environments/identities/managers.py	62.50%	3 Missing ⚠️
api/environments/identities/views.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3038      +/-   ##
==========================================
+ Coverage   95.73%   95.88%   +0.14%     
==========================================
  Files        1046     1045       -1     
  Lines       30859    30677     -182     
==========================================
- Hits        29544    29414     -130     
+ Misses       1315     1263      -52     

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[matthewelwell]

@khvn26 it seems like this PR shares a commit with #3039 ?

[khvn26]

@matthewelwell This is correct. I'll prefer to merge #3039 first and then this one.

[khvn26]

Actually let's just merge this one, there's no value in doing them separately.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ error: global flags not at the start of the expression at position 1 /api/v1/identities/ View Issue

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