feat: support copying referrers for multi-arch images

[qweeah]

What this PR does / why we need it:
This PR changes oras cp to recursively copy referrers of platform-specific images in an index or manifest list.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Resolves #816

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-commenter]

Codecov Report

Attention: Patch coverage is 70.96774% with 18 lines in your changes missing coverage. Please review.

Project coverage is 80.10%. Comparing base (0efe794) to head (cd84321).
Report is 246 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/oras/root/cp.go	70.00%	8 Missing and 4 partials ⚠️
internal/graph/graph.go	72.72%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1122      +/-   ##
==========================================
- Coverage   80.40%   80.10%   -0.31%     
==========================================
  Files          57       57              
  Lines        2945     2991      +46     
==========================================
+ Hits         2368     2396      +28     
- Misses        398      410      +12     
- Partials      179      185       +6     

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[shizhMSFT]

LGTM

[shizhMSFT]

LGTM