extend metadata schema to provide a field for custom entries

[RafalSkolasinski]

It seems that our metadata schema is over restrictive and we should explore extending it to allow users providing a custom set of parameters they would like to store as metadata.

Current suggestion is to allow one more top-level key, tags, that will store key-value pairs.
This would allow for storing arbitrary information like build number, training data identifier or training parameters.

$metadata_model_response =
{
  "name" : $string,
  "versions" : [ $string, ... ] #optional,
  "platform" : $string,
  "inputs" : [ $metadata_tensor, ... ],
  "outputs" : [ $metadata_tensor, ... ],
  "tags": { $string: $string, ....}
}

Example:

{
   ...
   "tags": {
      "build": 101,
      "training-data": "my-set-01",
   },
   ...
}

Note: this is about static /metadata endpoint and NOT the request tags

[RafalSkolasinski]

New new field will be called custom.