`oras manifest index create/update --output -` should hide other stdout output

[qweeah]

To be consistent with --format output, oras manifest index create/update --output - should hide other stdout outputs.

ORAS Version Version: 1.2.0-beta.1+unreleased Go version: go1.22.1 Git commit: c179d0a5ef6bd082318c2fc50856309c520e6031 Git tree state: clean

Behavior

> 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"
      }
    }
  ]
}

Wanted Behavior

> oras manifest index create myregistry.azurecr.io/test sha256:42c524c48e0672568dbd2842d3a0cb34a415347145ee9fe1c8abaf65e7455b46 --output - --pretty
{
  "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"
      }
    }
  ]
}