[CLI] Generate Controller with CRUD methods

[kjdelisle]

Story

As a loopback4 user, I would like to run a CLI command to generate a default set of CRUD API methods based on a model and its associated datasource(s) so that I can avoid hand-writing each of these functions manually.

Acceptance Criteria

• Prompt for controller name
• Validate controller name
• Prompt for whether or not they want to use the RestCrudController base template
• If RestCrudController:
  • Generate new class that extends this type (should be a very small template!)

    async create(obj: T) : Promise<T>
    count(where: Where) : Promise<number>
    find(filter: Filter) : Promise<T[]>
    updateAll(where: Where, obj: T) : Promise<number>
    deleteAll(where: Where): Promise<number>

    findById(id: ID) :  Promise<T>
    updateById(id: ID, obj: T) :  Promise<boolean>
    deleteById(id: ID) :  Promise<boolean>

• else
  • Fall back to generating an empty controller
• Document this feature on loopback.io
• Release the CLI package
• Write a blog post about how awesome this new feature is and how to use it.

[bajtos]

As far as this story is concerned, it's ok return 200 from the create (POST) method (but see #788).