[BUG][typescript-fetch] deepObject serialization not supported in query string

Created by: Dijky

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

A query parameter of type: object using additionalProperties produces Typescript code that fails to compile.

A similar parameter without additionalProperties that instead specifies properties compiles properly, but serializes the parameter improperly.

Typescript compiler error with `additionalProperties`

api/test/apis/DefaultApi.ts
Type error: Type '{ [key: string]: number; }' is not assignable to type 'string | number | boolean | (string | number | boolean)[]'.
  Type '{ [key: string]: number; }' is not assignable to type 'string'.  TS2322

    30 | 
    31 |         if (requestParameters.foffset !== undefined) {
  > 32 |             queryParameters['foffset'] = requestParameters.foffset;
       |             ^
    33 |         }
    34 | 
    35 |         const headerParameters: runtime.HTTPHeaders = {};

Invalid serialization with specified properties

The parameter is decoded to the string representation of an object in Javascript:

foffset=%5Bobject%20Object%5D (URL encoded)
foffset=[object Object] (URL decoded)

openapi-generator version

v4.0.0-beta2

As far as I can tell, there have been no relevant changes on master.

OpenAPI declaration file

openapi: 3.0.0
info:
  title: Test API
  version: 0.0.1
paths:
  /search:
    get:
      parameters:
        - name: foffset
          in: query
          required: false
          schema:
            type: object
            additionalProperties:
              type: integer
      responses:
        '200':
          description: OK

Command line used for generation

openapi-generator generate -g typescript-fetch -i ./test.openapi.yml -o api/

Steps to reproduce

Specify a query parameter of type: object.
Generate typescript-fetch client code.
Include/compile the generated API typescript file (additionalProperties variant fails here).
Perform an API request providing an object for the parameter.

Related issues/PRs

Possibly related to PR #569 and Issue #802 (closed).

Suggest a fix

I'm working on a first pull request.
The idea is to allow nested HTTPQuery objects that are recursively serialized.