nydusify: support target backend for copy subcommand

[imeoer]

Overview

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Related Issues

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Change Details

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Test Results

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Change Type

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• Bug Fix
• Feature Addition
• Documentation Update
• Code Refactoring
• Performance Improvement
• Other (please describe)

Self-Checklist

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• I have run a code style check and addressed any warnings/errors.
• I have added appropriate comments to my code (if applicable).
• I have updated the documentation (if applicable).
• I have written appropriate unit tests.

[codecov]

Codecov Report

❌ Patch coverage is 9.72763% with 232 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.96%. Comparing base (460bb74) to head (293403c).

Files with missing lines	Patch %	Lines
contrib/nydusify/pkg/copier/copier.go	8.73%	227 Missing and 3 partials ⚠️
contrib/nydusify/cmd/nydusify.go	60.00%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (9.72%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (69.96%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1930      +/-   ##
==========================================
+ Coverage   66.13%   69.96%   +3.82%     
==========================================
  Files         200      200              
  Lines       55859    56858     +999     
  Branches    48639    49403     +764     
==========================================
+ Hits        36941    39779    +2838     
+ Misses      17251    15133    -2118     
- Partials     1667     1946     +279     

Files with missing lines	Coverage Δ
contrib/nydusify/cmd/nydusify.go	69.03% <60.00%> (+52.13%)	⬆️
contrib/nydusify/pkg/copier/copier.go	21.17% <8.73%> (+6.89%)	⬆️

... and 35 files with indirect coverage changes

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