Add navigational properties to examples/todo-list

[bajtos]

Update examples/todo-list to leverage recently added features moving us closer to full inclusion of related models.

See the spike #2592 for more details.

Acceptance criteria

• Define {Model}Relations interfaces and {Model}WithRelations types

• Update {Model}Repository implementations to use these new interfaces

• Update repositories to include related models: overwrite find and findById methods, add a hard-coded retrieval of related models. This part depends on Modify Repository find* methods to include navigational properties #2632

  Include a comment explaining that the example implementation is temporary and add a link to the GitHub issue tracking the work needed to implement production-grade inclusion resolver.

• Add test coverage at repository level (integration tests)

• Update response schemas for controller methods find and findById to leverage getModelSchemaRef and includeRelations. This part depends on Implement getJsonSchemaRef and getModelSchemaRef helpers #2631 and Enhance getJsonSchema to describe navigational properties #2630

• Add acceptance-level tests to verify inclusion of related models via REST API

• Update Todo List Tutorial