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:

• [x] 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[bot]]

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 :warning:
cmd/oras/root/manifest/index/create.go	50.00%	10 Missing and 5 partials :warning:

Additional details and impacted files

```diff @@ 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.