fix: LEAP-884: avoid reading entire files to memory in export api

[jombooth]

Resolves #5243

High level idea is to avoid calling f.read() in the ExportAPI, using shutil.copy2 instead where we can.

Tested:

• with CI, for single file exports. eg pytest tests/sdk/test_tasks.py::test_export_tasks
• manually, for exporting an archive
• using label-studio export, for the tasks.functions change
• using pytest tests/data_export.tavern.yml, for the export mixin change.

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[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 75.93%. Comparing base (066f74b) to head (100bfbb).
Report is 2 commits behind head on develop.

Files	Patch %	Lines
label_studio/data_export/models.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5612   +/-   ##
========================================
  Coverage    75.92%   75.93%           
========================================
  Files          158      158           
  Lines        13359    13363    +4     
========================================
+ Hits         10143    10147    +4     
  Misses        3216     3216           

Flag	Coverage Δ
pytests	75.93% <95.00%> (+<0.01%)	⬆️

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[jombooth]

With the Windows failures, we can't merge this as is. Finding a solution will require a bit more time.

Update 4/24: seems to be fixed via Django's NamedTemporaryFile