Constants as openapi const union type

[apasquini95]

Hi everyone, I'm opening this PR as an enhancement of the previous one (#1086). I introduced the const schema also when handling union types, but only if the literal has only one value.

Union type with single literal value:

class MyController {
  const MY_CONST = "my_const";

  public function index(bool $param): string|MyDto
  {
    return $param ? self::MY_CONST : new MyDto;
  }
}

anyOf:
    - { $ref: '#/components/schemas/MyDto' }
    - { type: string, const: 'my_const' }

Union type with more than one literal value:

class MyController {
  const MY_CONST = "my_const";
  const MY_OTHER_CONST = "my_other_const";

  public function index(bool $param1, bool $param2): string|MyDto
  {
    return $param1 
      ? self::MY_CONST 
      : ($param2
        ? self::MY_OTHER_CONST
        : new MyDto);
  }
}

anyOf:
    - { $ref: '#/components/schemas/MyDto' }
    - { type: string, enum: ['my_const', 'my_other_const'] }

I decided to leave the enum schema when there are more than one literal of the same type just to avoid having a much longer result and keep its readability.

anyOf:
    - { $ref: '#/components/schemas/MyDto' }
    - { type: string, const: 'my_const' }
    - { type: string, const: 'my_other_const' }

[romalytvynenko]

@apasquini95 thanks for the pr, but this is not something I want to be a default behavior.

If you need this for your projects, consider using a document transformer and manually patch schemas there (there is an open api object traverser that can be handy for this task).