Generate OpenAPI documentation for API action

[Aschen]

Feature description

Allows to declare OpenAPI documentation for controller actions.

This documentation will be declared alongside the handler and http properties of an action:

app.controller.register('greeting', {
  actions: {
    sayHello: {
      handler: request => `Hello, ${request.input.args.name}`,
      // Open API documentation
      openApi: { ... } 
    }
  }
});

Then the complete API specification will be available to a new controller action (e.g. server:openApi)

Refactor

Existent native controller should be refactored to use the same controller definition as Kuzzle as a Framework so we can declare the open API specification

[Aschen]

Done