merge required constraints with allOf

[hauner]

support

FooRequired:
  allOf:
    - type: object
      required:
        - foo
    - $ref: '#/types/Foo'

types:
  Foo:
    type: object
    properties:
      foo:
        type: string

to generate

@Generated(value = "openapi-processor-spring", ...)
public class FooRequired {

    @NotNull
    @JsonProperty("foo")
    private String foo;

  // ...
}

[hauner]

already works