Improve defineCrudRestController to create a named controller class

[bajtos]

This is a follow-up for the spike #3617.

Improve @loopback/rest-crud - modify defineCrudRestController to create a named controller class, where the name is based on the model name we are creating the controller for.

Example implementation:

https://github.com/strongloop/loopback-next/blob/2848a1b3e4c62da88cfd3edc966e8267768cf326/packages/rest-crud/src/crud-rest.controller.ts#L260-L267

Acceptance criteria

• Implementation
• Automated tests
• Update API docs
• Update the example code in rest-crud README

[hacksparrow]

Closed via #3842