Define a data model and API structure for the license inventory

[JamieSlome]

To implement a license inventory, regardless of its ability to act as a standalone or integrated service, a data model and/or API model is required. More more information on the purpose of the license inventory, refer to #594.

### Tasks
- [x] Define a complete data model for the license inventory
- [x] Make use of SPDX IDs in the model
- [x] Refer to [choosealicense.com](https://choosealicense.com/) for data properties to support