Add support for generating pointers as x-nullable

[Thiht]

Describe the PR

With this change, pointers inside a struct are automatically generated with the x-nullable extension

New CLI flag: --nullablePointers

• Pointers in structs are generated with x-nullable: true
• If the schema contains a single $ref, like this:

my_field:
  $ref: "#/definitions/api.Child"
  x-nullable: true

it will be generated like this instead:

my_field:
  allOf:
    $ref: "#/definitions/api.Child"
  x-nullable: true

This ensures the result can then be converted to OpenAPI 3.0 without losing the nullable field (using: https://converter.swagger.io). See: https://stackoverflow.com/a/48114924

I'm willing to make some improvements if you have any suggestions, I'm not sure I made the change correctly as it just fits my use case for now :)

Relation issue
none

Additional context

I want to generate x-nullable on my pointers but I don't want to annotate all my fields with extensions:x-nullable. I think it makes sense to allow generating the x-nullable by default for all pointers.