[C][Client] Support a custom data type (IntOrString)

Created by: ityuhui

There is a custom data type from Kubernetes swagger specification:

   "description": "IntOrString is a type that can hold an int32 or a string.  When used in JSON or YAML marshalling and unmarshalling, it produces or consumes the inner type.  This allows you to have, for example, a JSON field that can accept a name or number.",
      "format": "int-or-string",
      "type": "string"
    },

And the model log (-DdebugModels) of openapi-generator-cli shows:

{
      "openApiType" : "string",
      "baseName" : "targetPort",
      "complexType" : "int_or_string",
      "getter" : "getTargetPort",
      "setter" : "setTargetPort",
      "description" : "IntOrString is a type that can hold an int32 or a string.  When used in JSON or YAML marshalling and unmarshalling, it produces or consumes the inner type.  This allows you to have, for example, a JSON field that can accept a name or number.",
      "dataType" : "int_or_string",
      "datatypeWithEnum" : "int_or_string",
      "dataFormat" : "int-or-string",
      "name" : "target_port",
      "defaultValueWithParam" : " = data.targetPort;",
      "baseType" : "int_or_string",
      "unescapedDescription" : "IntOrString is a type that can hold an int32 or a string.  When used in JSON or YAML marshalling and unmarshalling, it produces or consumes the inner type.  This allows you to have, for example, a JSON field that can accept a name or number.",
      "example" : "\"0\"",
      "jsonSchema" : "{\n  \"type\" : \"string\",\n  \"description\" : \"IntOrString is a type that can hold an int32 or a string.  When used in JSON or YAML marshalling and unmarshalling, it produces or consumes the inner type.  This allows you to have, for example, a JSON field that can accept a name or number.\",\n  \"format\" : \"int-or-string\"\n}",
      "exclusiveMinimum" : false,
      "exclusiveMaximum" : false,
      "required" : false,
      "deprecated" : false,
      "hasMoreNonReadOnly" : false,
      "isPrimitiveType" : false,
      "isModel" : false,
      "isContainer" : false,
      "isString" : true,
      "isNumeric" : false,
      "isInteger" : false,
      "isShort" : false,
      "isLong" : false,
      "isUnboundedInteger" : false,
      "isNumber" : false,
      "isFloat" : false,
      "isDouble" : false,
      "isDecimal" : false,
      "isByteArray" : false,
      "isBinary" : false,
      "isFile" : false,
      "isBoolean" : false,
      "isDate" : false,
      "isDateTime" : false,
      "isUuid" : false,
      "isUri" : false,
      "isEmail" : false,
      "isNull" : false,
      "isFreeFormObject" : false,
      "isAnyType" : false,
      "isArray" : false,
      "isMap" : false,
      "isEnum" : false,
      "isReadOnly" : false,
      "isWriteOnly" : false,
      "isNullable" : false,
      "isSelfReference" : false,
      "isCircularReference" : false,
      "isDiscriminator" : false,
      "vars" : [ ],
      "requiredVars" : [ ],
      "vendorExtensions" : { },
      "hasValidation" : false,
      "isInherited" : false,
      "nameInCamelCase" : "TargetPort",
      "nameInSnakeCase" : "TARGET_PORT",
      "uniqueItems" : false,
      "isXmlAttribute" : false,
      "isXmlWrapped" : false,
      "additionalPropertiesIsAnyType" : false,
      "hasVars" : false,
      "hasRequired" : false,
      "hasDiscriminatorWithNonEmptyMapping" : false,
      "hasMultipleTypes" : false,
      "datatype" : "int_or_string",
      "iexclusiveMaximum" : false
    } 

So this is a data type with below attributes:

• isContainer: false
• isPrimitiveType: false
• isModel: false
• isFreeFormObject: false

I add the support for this data type in C generator.

BTW: The header and implementation files of the custom data type cannot be provided by this PR because they need to be implemented by users themselves.

Hi @wing328 @zhemant @michelealbano

Could you please reivew this code change ? Thanks !

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