Backend: Basic CRUD REST Endpoint for units in libraries

[bradenmacdonald]

Allow creating a unit, viewing units in library, updating the title, and deleting the unit.

More Requirements:

1. Units must be added to the Meilisearch index and retrievable from the search APIs.
2. Enforce permissions, same as for creating a component in the library.
3. Set a "friendly" slug based on the title.

Out of scope:

1. No ability to add components to the unit yet.

[bradenmacdonald]

PR: openedx/openedx-platform#36371