allow users to pass neptune data directory as env var

[AleksanderWWW]

Before submitting checklist

• Did you update the CHANGELOG? (not for test updates, internal changes/refactors or CI/CD setup)
• Did you ask the docs owner to review all the user-facing changes?

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -4.16% ⚠️

Comparison is base (d48d235) 79.52% compared to head (b10b98f) 75.36%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1409      +/-   ##
==========================================
- Coverage   79.52%   75.36%   -4.16%     
==========================================
  Files         280      280              
  Lines       13874    13875       +1     
==========================================
- Hits        11033    10457     -576     
- Misses       2841     3418     +577     

Flag	Coverage Δ
e2e-management	?
e2e-s3	?
e2e-s3-gcs	?
e2e-standard	?
macos	72.80% <100.00%> (-6.43%)	⬇️
py3.7	73.10% <100.00%> (-5.87%)	⬇️
py3.9	?
ubuntu	72.96% <100.00%> (-6.34%)	⬇️
unit	73.10% <100.00%> (-0.62%)	⬇️
windows	72.24% <100.00%> (-7.03%)	⬇️

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Files Changed	Coverage Δ
.../operation_processors/async_operation_processor.py	87.16% <100.00%> (-0.18%)	⬇️
...peration_processors/offline_operation_processor.py	92.85% <100.00%> (ø)
...internal/operation_processors/operation_storage.py	94.28% <100.00%> (-2.49%)	⬇️
...l/operation_processors/sync_operation_processor.py	91.89% <100.00%> (-0.22%)	⬇️

... and 59 files with indirect coverage changes

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[AleksanderWWW]

This func is basically the clue of this PR. The rest is just adjusting the existing code for its usage