Proposal: Extend the example JSON definition to Assets

[fmigneault]

For the Machine Learning Model (MLM) extension, one of the Assets consist of the "source code" required to run the model training or inference pipeline. Provided that this definition is in itself a great example on how to run the model, it would make sense to mark it as such.

The current definition of the "source code" is as follows:

{
  "assets": {
    "source-training": {
      "href": "https:/github.com/org/repo/app.py",
      "type": "text/x-python",
      "title": "Source code to train the ML model.",
      "roles": [
        "ml-model:training-runtime",
        "mlm:model",
        "code",
        "metadata"
      ],
      "mlm:entrypoint": "app:train"
    }
  }
}

As we can see, there are strong resemblances to the example links, since details such as example:container and example:language would be relevant here as well. However, in the context of MLM, this script cannot be "only" considered an rel: example, since it is the definition that really tells us how to run it. An Asset seems more appropriate, since it is core to using the model itself.

While defining this "source code asset", similar consideration to the example:container field were also brought up (see https://github.com/crim-ca/dlm-extension/blob/validate/README.md#source-code-asset). If possible, I would like to leverage the example fields directly under the asset, instead of reinventing them with mlm: prefix. Another field that was relevant in our opinion is mlm:entrypoint, in case the referenced code has more than one function. This could be added to the exemple extension as well.