Allow a way to disable the use of O_DIRECT in parallel downloads

[sethiay]

Description

Allow a way to disable the use of O_DIRECT in parallel downloads

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - Tried the flag during perf testing.
2. Unit tests - Added.
3. Integration tests - TBA in existing e2e tests in a separate PR

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.03%. Comparing base (4638976) to head (e1b6740).
Report is 106 commits behind head on master.

Files with missing lines	Patch %	Lines
cfg/config.go	20.00%	2 Missing and 2 partials ⚠️
...al/cache/file/downloader/parallel_downloads_job.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2372      +/-   ##
==========================================
+ Coverage   79.01%   79.03%   +0.01%     
==========================================
  Files         105      105              
  Lines       11575    11584       +9     
==========================================
+ Hits         9146     9155       +9     
+ Misses       1963     1962       -1     
- Partials      466      467       +1     

Flag	Coverage Δ
unittests	79.03% <57.14%> (+0.01%)	⬆️

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