fix: Fix schema generation for oneOf when using TS-FETCH client

[giovannicimolin]

I'm using Django REST Framework + DRF Spectacular to automatically generate OpenAPI 3 schemas for my APIs.
When defining nullable enums, the API generator typescript-fetch creates code that doesn't compile properly with two errors:

• Spread types may only be created from object types (on FromJSONTyped functions).
• Module '"./BlankEnum"' has no exported member 'instanceOfBlankEnum' (enum templaces don't export function used by other API client files).

There's a problem generating typescript-fetch API clients using schemas containing oneOf and Enums.
Some parts of the API client seem to rely on the instaceOf{{classname}}, not provided for enum models.
Also, the code tries to create spread types from function types, leading to a TS compilation error.

I'm not sure this is the best way to fix the issue, my OpenAPI generator internals knowledge is somewhat limited.

Testing instructions

1. Check out this branch.
2. Build the generator.
3. Generate an API client using the following schema:

openapi: 3.0.3
info:
  title: Some API
  version: 1.0.0
  description: Some API documentation (internal).
paths:
  /api/v1/some-api/{id}/:
    patch:
      operationId: some_api_partial_update
      description: A viewset to edit some api information
      parameters:
      - in: path
        name: id
        schema:
          type: integer
        required: true
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/SomeRequest'
      security:
      - TokenAuthentication: []
      - basicAuth: []
      - cookieAuth: []
      responses:
        '200':
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/SomeRequest'
          description: ''
components:
  schemas:
    SomeRequest:
      type: object
      description: Some item serializer.
      properties:
        item_type:
          nullable: true
          oneOf:
          - $ref: '#/components/schemas/ItemTypeEnum'
          - $ref: '#/components/schemas/BlankEnum'
          - $ref: '#/components/schemas/NullEnum'
    BlankEnum:
      enum:
      - ''
    ItemTypeEnum:
      enum:
      - A
      - B
      - C
      type: string
    NullEnum:
      enum:
      - null

3. Create new node env, install typescript and run tsc (target: es2017).
4. Check that no type errors are reported by Typescript.

Related issues

• [BUG] [typescript-fetch] Module '"./MyEnum"' has no exported member 'instanceOfMyEnum'. and Spread types may only be created from object types. #15473
• [BUG] [typescript-fetch] oneOf containing enum tries to import nonexistent instanceOf function #14611
• [BUG] [TypeScript-Fetch] Incorrectly generated models when using oneOf #14763

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[giovannicimolin]

Pinging the members of the Typescript technical committee:

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

[macjohnny]

LGTM

thanks for your contribution