refactor: status, metadata and content handlers for manifest index commands

[wangxiaoxuan273]

What this PR does / why we need it:
Implements output handlers for manifest index commands.

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 #1503

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 82.52427% with 36 lines in your changes missing coverage. Please review.

Project coverage is 83.96%. Comparing base (b4be9a6) to head (2f1dd59).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/oras/root/manifest/index/update.go	61.11%	14 Missing and 7 partials ⚠️
cmd/oras/root/manifest/index/create.go	50.00%	10 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1509      +/-   ##
==========================================
- Coverage   83.99%   83.96%   -0.04%     
==========================================
  Files         118      119       +1     
  Lines        5174     5306     +132     
==========================================
+ Hits         4346     4455     +109     
- Misses        588      604      +16     
- Partials      240      247       +7     

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[wangxiaoxuan273]

As per offline discussion, added, merged, removed, pushed, packed should exist in both status and metadata output. And Status output is needed in the PR, metadata output can be added in a future pr when needed.

[shizhMSFT]

LGTM

[qweeah]

LGTM