[BUG][typescript-fetch] multipart/form-data request body does not respect date-time format

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

We are using openapi-generator-cli to generate typescript-fetch code based on an openapi 3.x spec generated by Swashbuckle. When the specification contains a path with a request body content type of multipart/form-data the resulting generated code will look like

if (requestParameters.someDate !== undefined) {
  formParams.append('someDate', requestParameters.someDate as any);
}

regardless of any format values provided in the specification. When the content type is application/json format values are honored. For example a property with a date-time format will call value.someDate.toISOString() when constructing the request as part of the <ModelName>ToJSON function

openapi-generator version

{
  "$schema": "node_modules/@openapitools/openapi-generator-cli/config.schema.json",
  "spaces": 2,
  "generator-cli": {
    "version": "4.3.1"
  }
}

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {
    "title": "Swashbuckle",
    "description": "Swagger document by Swashbuckle",
    "version": "v1"
  },
  "servers": [
    {
      "url": "https://someapi.com"
    }
  ],
  "paths": {
    "/api/example/": {
      "post": {
        "requestBody": {
          "content": {
            "multipart/form-data": {
              "schema": {
                "$ref": "#/components/schemas/ExampleBody"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ExampleResponse"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "ExampleBody": {
        "type": "object",
        "properties": {
          "someBoolean": {
            "type": "boolean"
          },
          "someDate": {
            "type": "string",
            "format": "date-time"
          },
          "someNumber": {
            "type": "number",
            "format": "double"
          },
          "someUuid": {
            "type": "string",
            "format": "uuid"
          },
          "someBinary": {
            "type": "string",
            "format": "binary",
            "nullable": true
          }
        },
        "additionalProperties": false
      },
      "ExampleResponse": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string",
            "format": "uuid"
          },
          "someBoolean": {
            "type": "boolean"
          },
          "someDate": {
            "type": "string",
            "format": "date-time"
          },
          "someNumber": {
            "type": "number",
            "format": "double"
          },
          "someUuid": {
            "type": "string",
            "format": "uuid"
          },
          "someBinary": {
            "type": "string",
            "format": "binary",
            "nullable": true
          }
        },
        "additionalProperties": false
      }
    }
  }
}

Generation Details

openapi-generator-cli generate -i openapi.json -o ./src --generator-name typescript-fetch --additional-properties=typescriptThreePlus=true,supportsES6=true

Steps to reproduce

1. create a openapi.json file with the contents above
2. execute the open api generator command above to generate typescript code
3. observe the generated code constructs form params with code like formParams.append('someDate', requestParameters.someDate as any); for the someDate property even though someDate has a format attribute.

Related issues/PRs

Suggest a fix

The when appending form data properties the generated code should inspect the format attribute if provided and provide any necessary transforms

conceptually

switch(format) {
  case 'date-time':
    formParams.append({propertyName} requestParameters.{propertyName}.toISOString());
    break;
  case 'other-format':
     ...
  default:
    formParams.append({propertyName} requestParameters.{propertyName} as any);
} 

Run the provided openapi-generator-cli command with the embedded openapi.json and inspect the generated typescript-fetch multipart request code. Trace the typescript-fetch generator's multipart form-parameter handling and add coverage for the date-time field; done means generated form data serializes someDate with the expected date-time representation while other fields remain correct.