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

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.

Start with the generate command using the supplied petstore.yaml and -DdebugModels to inspect the schema model for the deprecated tag field. Compare the existing isDeprecated value in api.mustache with the missing value needed by model.mustache. Done means the debug model exposes isDeprecated and model.mustache can use it for deprecated fields.