Add AMBAR dataset importer and CLI support

[sushobhan2024]

Add support for importing the AMBAR dataset into LensKit.

This includes:

• Add a load_ambar() dataset loader that reads the AMBAR dataset and returns a Dataset.
• Add CLI support so AMBAR can be imported using the LensKit command-line tools.
• Update the CLI so imported dataset can be described with the existing describe command.