[REQ] Add `isDeprecated` field to schema object.

Created by: yuji38kwmt

Is your feature request related to a problem? Please describe.

I created the following yaml file that tag field of Pet Object is deprecated. I executed generate command with -DdebugModels option and I printed JSON model. But json model does not include isDeprecated field. So I can not use isDrecated variable in model.mustache file.

petstore.yaml

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Swagger Petstore
  license:
    name: MIT
servers:
  - url: http://petstore.swagger.io/v1
paths:
  /pets/{petId}:
    get:
      summary: Info for a specific pet
      operationId: showPetById
      tags:
        - pets
      parameters:
        - name: petId
          in: path
          required: true
          description: The id of the pet to retrieve
          schema:
            type: string
      responses:
        '200':
          description: Expected response to a valid request
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Pet"
components:
  schemas:
    Pet:
      type: object
      required:
        - id
        - name
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
        tag:
          type: string
          deprecated: true
          

Executed command

$  java -Dmodels   -DmodelTests=false -DmodelDocs=false -DdebugModels \
 -jar openapi-generator-cli-4.0.3.jar  generate \
  -g python -i petstore.yaml 

JSON model with -DdebugModels option

 {
      "openApiType" : "string",
      "baseName" : "tag",
      "getter" : "getTag",
      "setter" : "setTag",
      "dataType" : "str",
      "datatypeWithEnum" : "str",
      "name" : "tag",
      "defaultValueWithParam" : " = data.tag;",
      "baseType" : "str",
      "example" : "null",
      "jsonSchema" : "{\n  \"type\" : \"string\",\n  \"deprecated\" : true\n}",
      "exclusiveMinimum" : false,
      "exclusiveMaximum" : false,
      "hasMore" : false,
      "required" : false,
      "secondaryParam" : false,
      "hasMoreNonReadOnly" : false,
      "isPrimitiveType" : true,
      "isModel" : false,
      "isContainer" : false,
      "isString" : true,
      "isNumeric" : false,
      "isInteger" : false,
      "isLong" : false,
      "isNumber" : false,
      "isFloat" : false,
      "isDouble" : false,
      "isByteArray" : false,
      "isBinary" : false,
      "isFile" : false,
      "isBoolean" : false,
      "isDate" : false,
      "isDateTime" : false,
      "isUuid" : false,
      "isUri" : false,
      "isEmail" : false,
      "isFreeFormObject" : false,
      "isListContainer" : false,
      "isMapContainer" : false,
      "isEnum" : false,
      "isReadOnly" : false,
      "isWriteOnly" : false,
      "isNullable" : false,
      "isSelfReference" : false,
      "vendorExtensions" : { },
      "hasValidation" : false,
      "isInherited" : false,
      "nameInCamelCase" : "Tag",
      "nameInSnakeCase" : "TAG",
      "isXmlAttribute" : false,
      "isXmlWrapped" : false,
      "iexclusiveMaximum" : false,
      "datatype" : "str"
    } 

Describe the solution you'd like

Add isDeprecated field to schema object.

Additional context

isDeprecated variable is available in api.mustache file.