feat: DIA-2206: prefetch fields in data export

[pakelley]

---

Prefetch fields causing lots of db traffic during export

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.37%. Comparing base (20fc797) to head (3b3db0e).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/data_export/mixins.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7410      +/-   ##
===========================================
- Coverage    78.37%   78.37%   -0.01%     
===========================================
  Files          193      193              
  Lines        15613    15616       +3     
===========================================
+ Hits         12237    12239       +2     
- Misses        3376     3377       +1     

Flag	Coverage Δ
pytests	78.37% <50.00%> (-0.01%)	⬇️

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