Swift4: Add additional initializer for initializing model object with *only* required properties

[nashish]

Description

In a previous feature, you added init() methods that include all properties declared in the schema. My request is to provide an option for an init() method with only the required properties from the schema.

Swagger-codegen version

3.0.2

Swagger declaration file content or url

Foo:
      required:
      - id
      properties:
        id:  # the init() function would only have this property 
          type: string
        color:
          type: string

Related issues/PRs

#6641