Add defineCrudRepositoryClass - a helper to create a named repository class

[bajtos]

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

Add defineCrudRepositoryClass to @loopback/rest-crud, this function should create a named repository class for the given Model class.

Example implementation (notice wrong function name):

https://github.com/strongloop/loopback-next/blob/2848a1b3e4c62da88cfd3edc966e8267768cf326/packages/rest-crud/src/repository-builder.ts#L14-L36

Acceptance criteria

• Implementation
• Automated tests
• API docs
• Add a code example to rest-crud README

[hacksparrow]

Closed via #3867