Add xml support to openapi3

[skywing918]

1. add test cases based on Examples
2. add xml in OpenAPI3Schema type
3. update schema-emitter logic for support xml object

[wanlwanl]

if xmlName is undefined, prop.name = "bbb", jsonName == "aaa",
then we ignore json name.
is this expected?

[wanlwanl]

no need to add ref? simply use 2nd parameter is enough?

[skywing918]

@timotheeguerin as you mentioned, it need set the xml name in items . but seems the xml name is not available if items,$ref existed.

@name("XmlPet")
model Pet {
  tags: Tag[];
}
@name("XmlTag")
model Tag {
  name: string;
}

Pet:
  type: object
  required:
    - tags
  properties:
    tags:
      type: array
      items:
        $ref: '#/components/schemas/Tag'
        xml:
          name: XmlTag
      xml:
        wrapped: true
  xml:
    name: XmlPet

[timotheeguerin]

@timotheeguerin as you mentioned, it need set the xml name in items . but seems the xml name is not available if items,$ref existed.

@name("XmlPet")
model Pet {
  tags: Tag[];
}
@name("XmlTag")
model Tag {
  name: string;
}

Pet:
  type: object
  required:
    - tags
  properties:
    tags:
      type: array
      items:
        $ref: '#/components/schemas/Tag'
        xml:
          name: XmlTag
      xml:
        wrapped: true
  xml:
    name: XmlPet

You need to wrap the $ref in allOf like we do for nullable, description or other attributes

      properties:
        tags:
          type: array
          items:
            allOf:
              - $ref: '#/components/schemas/Tag'
            xml:
              name: XmlTag
          xml:
            wrapped: true