Merging artifacts Branch: Add discover comand

[qweeah]

This PR adds a new discover command, which can be used for searching referrer artifacts.

Resolves #382, related to #379

[Wwwsylvia]

Can we put a screenshot of the command demo in the comment?

[qweeah]

Can we put a screenshot of the command demo in the comment?

@Wwwsylvia Suppose we have

localhost:5000/net-monitor:v1
├── signature/example
│   └── sha256:f7a4c823ef97b046dd219bd78952f47e3794172650570d524953f8033670e0f2
│       └── signature/example
│           └── sha256:3516a004b42f1dce8902a16f97e5c88e9fa4992066815b45037dc28b1fdf7740
└── sbom/example
    └── sha256:ec564f5a4c71941928766f8d3e7c429063afcbc308a3dd1aa59bca147008408d
        └── signature/example
            └── sha256:7aed904b970f3fe9776a60cd047dc0b9c0efe20d65c242b8ac5625a95fc8ae81

• oras discover localhost:5000/net-monitor:v1 -o json will show

{
  "referrers": [
    {
      "digest": "sha256:f7a4c823ef97b046dd219bd78952f47e3794172650570d524953f8033670e0f2",
      "mediaType": "application/vnd.cncf.oras.artifact.manifest.v1+json",
      "artifactType": "signature/example",
      "size": 411
    },
    {
      "digest": "sha256:ec564f5a4c71941928766f8d3e7c429063afcbc308a3dd1aa59bca147008408d",
      "mediaType": "application/vnd.cncf.oras.artifact.manifest.v1+json",
      "artifactType": "sbom/example",
      "size": 401
    }
  ]
}

• oras discover localhost:5000/net-monitor:v1 will show

Discovered 2 artifacts referencing localhost:5000/net-monitor:v1
Digest: sha256:c33272f6cb9bd31fffed825757c9693ea19cb01a958a11ebb0fce86972f8ff0c

Artifact Type       Digest
signature/example   sha256:f7a4c823ef97b046dd219bd78952f47e3794172650570d524953f8033670e0f2
sbom/example        sha256:ec564f5a4c71941928766f8d3e7c429063afcbc308a3dd1aa59bca147008408d

• oras discover localhost:5000/net-monitor:v1 --artifact-type signature/example will show

localhost:5000/net-monitor:v1
└── signature/example
    └── sha256:f7a4c823ef97b046dd219bd78952f47e3794172650570d524953f8033670e0f2
        └── signature/example
            └── sha256:3516a004b42f1dce8902a16f97e5c88e9fa4992066815b45037dc28b1fdf7740

• oras discover localhost:5000/net-monitor:v1 --artifact-type sbom/example will show

localhost:5000/net-monitor:v1
└── sbom/example
    └── sha256:ec564f5a4c71941928766f8d3e7c429063afcbc308a3dd1aa59bca147008408d

[shizhMSFT]

Let's review the UX offline tomorrow.

[qweeah]

Added latest tag a default:

• Tree

Using default tag: latest
localhost:5000/net-monitor:latest
├── signature/example
│   └── sha256:2437d8720c28d2bd2ad830d218ebb47942c099e21ff9cf9abbeb5e396203f6dd
│       └── signature/example
│           └── sha256:7c8fa4053dd322a3c1ea069a43c2cfba90ef4db4c9fed71f10da9d3a2071479c
└── sbom/example
    └── sha256:4b6726d63d4283fa579ddf29c4b0838025e2e27e5856e25db78fb50387461ee3
        └── signature/example
            └── sha256:673048d3beb0fada31b268b0e56a89c49fd6c0d2ec9b5ba63330e712b0c6758f

• Table

Using default tag: latest
Discovered 2 artifacts referencing localhost:5000/net-monitor:latest
Digest: sha256:4d5dd672805a7e20d19d1ae89bf4c5d371c47a018cb33bbc4dec989444602bd0

Artifact Type       Digest
signature/example   sha256:2437d8720c28d2bd2ad830d218ebb47942c099e21ff9cf9abbeb5e396203f6dd
sbom/example        sha256:4b6726d63d4283fa579ddf29c4b0838025e2e27e5856e25db78fb50387461ee3

• JSON

Using default tag: latest
{
  "referrers": [
    {
      "digest": "sha256:2437d8720c28d2bd2ad830d218ebb47942c099e21ff9cf9abbeb5e396203f6dd",
      "mediaType": "application/vnd.cncf.oras.artifact.manifest.v1+json",
      "artifactType": "signature/example",
      "size": 411
    },
    {
      "digest": "sha256:4b6726d63d4283fa579ddf29c4b0838025e2e27e5856e25db78fb50387461ee3",
      "mediaType": "application/vnd.cncf.oras.artifact.manifest.v1+json",
      "artifactType": "sbom/example",
      "size": 401
    }
  ]
}

[shizhMSFT]

LGTM