chore: raw manifest indentation

[GabriFedi97]

Description

Add indentation to raw manifests to improve readability.

Example

Following the example in the issue #2055, mutating the manifest after this patch will produce a new manifest with indentation:

2. Inspect the manifest with crane manifest:

crane manifest localhost:5000/simple-manifest:latest

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.docker.container.image.v1+json",
    "size": 897,
    "digest": "sha256:701db07f8f291e50fbb06b4dd13310acad36c53120adbdf2568bd2009d48a2a3"
  },
  "layers": null
}

3. Mutate the manifest with crane mutate:

crane mutate localhost:5000/simple-manifest:latest

2025/02/08 13:04:41 existing blob: sha256:701db07f8f291e50fbb06b4dd13310acad36c53120adbdf2568bd2009d48a2a3
2025/02/08 13:04:41 localhost:5000/simple-manifest:latest: digest: sha256:e265ee69944da7c7dc2c32a18f11d2140333371a3fda5478a4900de543dacb45 size: 304
localhost:5000/simple-manifest@sha256:e265ee69944da7c7dc2c32a18f11d2140333371a3fda5478a4900de543dacb45

4. Inspect the manifest again with crane manifest:

crane manifest localhost:5000/simple-manifest:latest
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.docker.container.image.v1+json",
    "size": 897,
    "digest": "sha256:701db07f8f291e50fbb06b4dd13310acad36c53120adbdf2568bd2009d48a2a3"
  },
  "layers": null
}

Closes #2055