feat: --output for `oras manifest index create`

[wangxiaoxuan273]

What this PR does / why we need it:

current output:

oras manifest index create myregistry.azurecr.io/test sha256:42c524c48e0672568dbd2842d3a0cb34a415347145ee9fe1c8abaf65e7455b46 --output - --pretty
Fetching sha256:42c524c48e0672568dbd2842d3a0cb34a415347145ee9fe1c8abaf65e7455b46
Fetched  sha256:42c524c48e0672568dbd2842d3a0cb34a415347145ee9fe1c8abaf65e7455b46
Packed   f16df862b7e3 application/vnd.oci.image.index.v1+json
Digest: sha256:f16df862b7e3d4f13c03d59655ed25a71a68e01e4aeec0d921d7310d6c67a360
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:42c524c48e0672568dbd2842d3a0cb34a415347145ee9fe1c8abaf65e7455b46",
      "size": 1239,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    }
  ]
}

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Part of #1053

Please check the following list:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.83%. Comparing base (0baec35) to head (d7055ad). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1490 +/- ## ========================================== + Coverage 85.73% 85.83% +0.09% ========================================== Files 116 116 Lines 4059 4073 +14 ========================================== + Hits 3480 3496 +16 + Misses 347 345 -2 Partials 232 232 ```

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[qweeah]

Shouldn't --output - forbids status output?

[wangxiaoxuan273]

Shouldn't --output - forbids status output?

Is it necessary?

[qweeah]

I think it's intuitive and conventional. E.g. curl turns off status output when using -o -.