pkg/object: add copy interface

[zhijian-pro]

No description provided.

[github-actions]

Mutate Test Report
pkg/object/object_storage_test.go-1-1: score:0.17 failed:10, passed:2, compile error:1, out of coverage:0, skip by comment:0, others:0
Job detail: https://github.com/juicedata/juicefs/actions/runs/4935356922/jobs/8821594185

Usage: https://github.com/juicedata/juicefs/blob/main/.github/scripts/mutate/how_to_use_mutate_test.md

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (1770458) 55.98% compared to head (7b37ff6) 55.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3581      +/-   ##
==========================================
- Coverage   55.98%   55.98%   -0.01%     
==========================================
  Files         149      149              
  Lines       37031    37127      +96     
==========================================
+ Hits        20732    20784      +52     
- Misses      14020    14047      +27     
- Partials     2279     2296      +17     

Impacted Files	Coverage Δ
pkg/object/interface.go	66.66% <ø> (ø)
pkg/object/object_storage.go	76.92% <100.00%> (+0.73%)	⬆️
pkg/object/prefix.go	72.80% <100.00%> (+0.48%)	⬆️
pkg/object/sharding.go	62.59% <100.00%> (+0.57%)	⬆️

... and 17 files with indirect coverage changes

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