spec: Multi-arch image management with ORAS

[FeynmanZhou]

What this PR does / why we need it:

Add a spec: Multi-arch image management with ORAS

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 #1053

Preview: https://github.com/FeynmanZhou/oras/blob/multi-arch/docs/proposals/multi-arch-image-mgmt.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.45%. Comparing base (c24a11e) to head (824613f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1514   +/-   ##
=======================================
  Coverage   84.45%   84.45%           
=======================================
  Files         125      125           
  Lines        5661     5661           
=======================================
  Hits         4781     4781           
  Misses        625      625           
  Partials      255      255           

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[shizhMSFT]

To make the spec more professional, please proofread the spec and fix formatting issues.

[shizhMSFT]

LGTM but you need to fix the check failure.

[tarilabs]

thank you for drawing my attention to this, @FeynmanZhou ! 🙏

some very optional comments below, I hope these helps!

[Wwwsylvia]

LGTM

[FeynmanZhou]

Hi @oras-project/oras-cli , would you be able to merge this PR as the initial version? We can append changes for further enhancements in beta.3 and future releases.