Support x-nullable extension for swagger

[Stranger6667]

From https://help.apiary.io/api_101/swagger-extensions/#x-nullable:

As it is not possible to declare null as an additional type to schemas in Swagger 2 this brings a limitation where you cannot define types which are nullable. In OAS3 this limitation is removed with the introduction of the nullable property which when set to true allows null to be a value alongside the original value(s) or type(s). This feature is backported to our Swagger 2 parser as a vendored extension x-nullable.

For example, to declare a schema for a type that may be a string or null:

type: string
x-nullable: true

x-nullable may also be used in conjunction with enumerations. In the below example the schema represents that the permitted values are either one of the strings north, east, south, west or null:

enum:
  - north
  - east
  - south
  - west
x-nullable: true

It will be a valuable addition to support it since it is quite popular (at least in kiwi.com). It could be done with transforming relevant schema to correct JSON schema representation, for example with anyOf

{
  "anyOf": [
    { "type": "string", "maxLength": 5 },
    { "type": "null"}
  ]
}