[Feat] Support reconvert image from nydus to oci (Update for PR #677)

[J-jxr]

Overview

This PR is an update based on the original PR #677 (submitted by BraveY). It addresses some review comments from the original PR, retains the full commit history , and ensures the core function—reconverting nydus-format images to OCI format—works as expected.

Related Issues

PR#677
Related dragonflyoss/nydus#1682 (follows the same upstream issue as the original PR)

Change Details

1. Added defer cw.Close() for blob writer (cw)
2. Added "uncompressed" option to the Compressor parameter (default remains "gzip") to cover uncompressed OCI image scenarios
3. Cleaned up unused code: Removed unused comments (e.g., LayerAnnotationOciSourceDigest)
4. Optimized memory usage: Modified the logic for writing large OCI tar packages

Change Type

• Feature Addition (aligns with the original PR's feature type, focusing on completing the nydus-to-OCI reconversion feature)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 594 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.58%. Comparing base (0784b5a) to head (e249a49).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/reconverter/default.go	0.00%	386 Missing ⚠️
pkg/converter/convert_unix.go	0.00%	153 Missing ⚠️
pkg/reconverter/converter.go	0.00%	53 Missing ⚠️
pkg/cgroup/v1/v1.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
- Coverage   20.41%   19.58%   -0.84%     
==========================================
  Files         122      124       +2     
  Lines       13864    14452     +588     
==========================================
  Hits         2830     2830              
- Misses      10715    11303     +588     
  Partials      319      319              

Files with missing lines	Coverage Δ
pkg/converter/types.go	0.00% <ø> (ø)
pkg/cgroup/v1/v1.go	0.00% <0.00%> (ø)
pkg/reconverter/converter.go	0.00% <0.00%> (ø)
pkg/converter/convert_unix.go	0.00% <0.00%> (ø)
pkg/reconverter/default.go	0.00% <0.00%> (ø)

... and 8 files with indirect coverage changes

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[BraveY]

Thanks for the PR. LGTM!

[Zephyrcf]

LGTM!