fix: should not print copy hint for manifest in verbose mode

[qweeah]

What this PR does / why we need it:
This PR avoids wrong hinting on unnamed manifest/index in verbose mode.

Suppose we have below manifest stored in registry

$ oras manifest fetch --pretty localhost:5000/test:config                                                         
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "artifactType": "my.test/example",
  "config": {
    "mediaType": "bar/test",
    "digest": "sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae",
    "size": 3
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar",
      "digest": "sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae",
      "size": 3,
      "annotations": {
        "org.opencontainers.image.title": "foo"
      }
    }
  ],
  "annotations": {
    "org.opencontainers.image.created": "2023-11-03T01:20:50Z"
  }
}

Before the change, the verbose output will always hint user to use oras copy to copy unnamed layer. But actually only the manifest is unnamed

$ oras pull localhost:5000/test:config -o pulled --config my.config -v
✓ Pulled      my.config                                                                                                                                                                       3/3  B 100.00%    3ms
  └─ sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae 
✓ Pulled      foo                                                                                                                                                                             3/3  B 100.00%  706µs
  └─ sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae 
✓ Pulled      application/vnd.oci.image.manifest.v1+json                                                                                                                                  522/522  B 100.00%  262µs
  └─ sha256:007fa1a84c35baf14b14698b8f2f4b3eb99200b496be446e6ea6c478029d75b9 
Skipped pulling layers without file name in "org.opencontainers.image.title"
Use 'oras copy localhost:5000/test:config --to-oci-layout <layout-dir>' to pull all layers.

After the change, the verbose output will not do that

oras pull localhost:5000/test:config -o pulled --config my.config 
✓ Pulled      my.config                                                                                                                                                                       3/3  B 100.00%    1ms
  └─ sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae 
✓ Pulled      foo                                                                                                                                                                             3/3  B 100.00%    4ms
  └─ sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae 
✓ Pulled      application/vnd.oci.image.manifest.v1+json                                                                                                                                  522/522  B 100.00%     0s
  └─ sha256:007fa1a84c35baf14b14698b8f2f4b3eb99200b496be446e6ea6c478029d75b9 
Pulled [registry] localhost:5000/test:config
Digest: sha256:007fa1a84c35baf14b14698b8f2f4b3eb99200b496be446e6ea6c478029d75b9

[codecov-commenter]

Codecov Report

Merging #1165 (cf5d282) into main (f41d56d) will decrease coverage by 0.01%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #1165      +/-   ##
==========================================
- Coverage   79.94%   79.93%   -0.01%     
==========================================
  Files          65       65              
  Lines        3520     3519       -1     
==========================================
- Hits         2814     2813       -1     
  Misses        492      492              
  Partials      214      214              

Files	Coverage Δ
cmd/oras/root/pull.go	73.78% <ø> (-0.13%)	⬇️

[shizhMSFT]

LGTM