Add a field for arbitrary metadata in annotation

[aguschin]

For some, it can be useful to keep arbitrary metadata. Since artifacts.yaml is a yaml file, it could be yaml or anything that works. Something like:

mymodel:
  path: path/to/model
  extra: <.......arbitrary metadata.....>

[aguschin]

Some options for the name: extra, custom, info, notes, meta, details. Example with custom:

$ cat example-gto/artifacts.yaml
awesome:
  description: This model is very cool
  labels:
  - ml
  - cool
  path: models/awesome.pkl
  type: model
  custom: 1