refactor: improve metadata handler of oras cp

[wangxiaoxuan273]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1548 , which was falsely marked as closed
Part of #1542

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.33%. Comparing base (ccd9f3f) to head (2b8e4b6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/oras/root/cp.go	25.00%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (66.66%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1624      +/-   ##
==========================================
- Coverage   84.37%   84.33%   -0.05%     
==========================================
  Files         120      120              
  Lines        5460     5464       +4     
==========================================
+ Hits         4607     4608       +1     
- Misses        606      608       +2     
- Partials      247      248       +1     

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[shizhMSFT]

LGTM