Add support for oneOf inheritance

[glopesdev]

This PR adds preliminary support for validation and code generation of oneOf inheritance patterns where discriminated unions are used to define polymorphic properties or arrays. In this case, it is common to use a oneOf restriction to specify the exact set of allowed types.

NJsonSchema does not directly support this yet (see RicoSuter/NJsonSchema#13), but the approach detailed here works for some of the most common cases. Supporting this pattern will allow code completion of polymorphic types (with discriminator validation) for manually edited YAML / JSON files.

An example of a supported schema is below:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "title": "Container",
  "properties": {
    "Animals": {
      "type": "array",
      "items": { "$ref": "#/definitions/AnimalTypes" }
    }
  },
  "definitions": {
    "Dog": {
      "type": "object",
      "properties": {
        "kind": {
          "enum": [ "Dog" ]
        },
        "Bar": {
          "type": [
            "null",
            "string"
          ]
        }
      },
      "required": [ "kind", "Bar" ],
      "allOf": [
        {
          "$ref": "#/definitions/Animal"
        }
      ]
    },
    "Cat": {
      "type": "object",
      "properties": {
        "kind": {
          "enum": [ "Cat" ]
        },
        "Baz": {
          "type": [
            "null",
            "string"
          ]
        }
      },
      "required": [ "kind", "Baz" ],
      "allOf": [
        {
          "$ref": "#/definitions/Animal"
        }
      ]
    },
    "Animal": {
      "type": "object",
      "discriminator": "kind",
      "x-abstract": true,
      "required": [
        "kind"
      ],
      "properties": {
        "Foo": {
          "type": [
            "null",
            "string"
          ]
        },
        "kind": {
          "type": "string"
        }
      }
    },
    "AnimalTypes": {
      "oneOf": [
        {
          "$ref": "#/definitions/Dog"
        },
        {
          "$ref": "#/definitions/Cat"
        },
        {
          "type": "null"
        }
      ]
    }
  }
}