Support for unsupervised and semisupervised learning

[duckontheweb]

An ml-aoi Item must link to exactly one STAC item that is using label extension.

This requirement means that the extension can only be used to describe training data for supervised learning tasks. This issue exists to:

1. Discuss whether we should drop this requirement in order to support unsupervised and/or semi-supervised learning tasks
2. Discuss what other changes would be necessary to support these tasks.