feat(simple reader): Implementing read using new MRD Simple Reader

[abhishek10004]

Description

This pull request introduces a new 'MRD Simple Reader' for zonal buckets, enabled by a new enable-kernel-reader flag. This involves adding a new read path in the filesystem logic, and several new components like MrdInstance and MrdSimpleReader to manage multi-range downloads. The changes are extensive and include new logic for caching and lifecycle management of downloader instances, along with comprehensive tests.

Link to the issue in case of a bug fix.

b/472227021

Testing details

1. Manual - Done
2. Unit tests - Done
3. Integration tests - Will be part of a separate PR

Any backward incompatible change? If so, please explain.

[github-actions]

/gemini review

[github-actions]

/gemini summary

[codecov]

Codecov Report

❌ Patch coverage is 79.34783% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.05%. Comparing base (3d0a0b9) to head (0835d22).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/gcsx/mrd_instance.go	83.72%	12 Missing and 9 partials ⚠️
internal/fs/fs.go	0.00%	6 Missing and 1 partial ⚠️
internal/gcsx/mrd_pool.go	22.22%	6 Missing and 1 partial ⚠️
internal/fs/handle/file.go	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4234      +/-   ##
==========================================
- Coverage   83.10%   83.05%   -0.06%     
==========================================
  Files         154      156       +2     
  Lines       18915    19157     +242     
==========================================
+ Hits        15720    15911     +191     
- Misses       2622     2653      +31     
- Partials      573      593      +20     

Flag	Coverage Δ
unittests	83.05% <79.34%> (-0.06%)	⬇️

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