Switch to sigs.k8s.io/yaml instead of default; fixup jozufile struct

[amisevsk]

Description

• Switch to using sigs.k8s.io/yaml for YAML serialization/deserialization to avoid ambiguity between JSON and YAML representations of the Jozufile.
• Add the omitempty tag to fields, to avoid serializing empty structs, and converts struct fields to pointers to ensure they can be nil/empty
• Fixup struct fields; we were missing .package.license from the struct, even though it's present in the sample.

More info

gopkg.in/yaml.v3 uses a separate struct tag for YAML fields, leaving ambiguity between the JSON and YAML serialized format of a struct, unless struct tags are repeated:

type myStruct struct {
  myfield string `json:"myfield,omitempty",yaml:"myfield,omitempty"`
}

sigs.k8s.io/yaml instead reuses json struct tags, allowing for one specification to apply to both formats.

Prior to this PR, the jozufile.MarshalToJSON method would use capitalized fields instead of lowercase:

Before

{
  "ManifestVersion": "1.0",
  "Package": {
    "Name": "Densenet-ONNX",
    "Version": "",
    "Description": "Example using onnx.",
    "Authors": [
      "Gorkem"
    ]
  },
  "Code": null,
  "DataSets": null,
  "Models": [
    {
      "Name": "onnx Model",
      "Path": "./model",
      "Framework": "sklearn:1.3.0",
      "Version": "1.2",
      "Description": "Example model using onnx",
      "License": "Apache-2.0",
      "Training": {
        "DataSet": "",
        "Parameters": null
      },
      "Validation": {
        "DataSet": "",
        "Metrics": null
      }
    }
  ]
}

After

{
  "manifestVersion": "1",
  "package": {
    "name": "Densenet-ONNX",
    "description": "Example using onnx.",
    "license": "Apache-2.0",
    "authors": [
      "Gorkem"
    ]
  },
  "models": [
    {
      "name": "onnx Model",
      "path": "./model",
      "framework": "sklearn:1.3.0",
      "version": "1.2",
      "description": "Example model using onnx",
      "license": "Apache-2.0"
    }
  ]
}

[amisevsk]

Note this change probably invalidate existing built artifacts; configs (and thus manifests) will change here. Old stores should be cleared after this PR is merged: rm -rf ~/.jozu/storage