[C++] [Qt5] Add initial version of File upload and download for Qt5 client

Created by: etherealjoy

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
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Description of the PR

• Add File Upload support for the Qt5 client
• Add File download support for the Qt5 client
• Setup file handling for the Qt5 server only for the common part with client

ToDo:

• Setup file handling for the Qt5 server
• The HttpFileElement can be extended to pass QIODevice streams instead which includes files, sockets and so on.

Fixes: #3651 #3421 (closed) #3457 (closed)

Note: Not a breaking change for the File handling changes, as it never worked to begin with.

Sample Usage:

#include <QDebug>
#include <QCoreApplication>
#include <QTimer>
#include <OAIDefaultApi.h>

using namespace OpenAPI;

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);
    OAIDefaultApi api;
    api.setHost("http://127.0.0.1:8081");

    OpenAPI::OAIHttpFileElement fileElement;
    fileElement.setMimeType("image/png;charset=UTF-8");
    fileElement.setRequestFileName("FileExt2.png");
    fileElement.setFileName("/home/dev/testapp/ext2.png");
    QString fileId = "fileId";
    qint32 orderId = 555;
    qint32 userId = 777;

    QObject::connect(&api, &OpenAPI::OAIDefaultApi::downLoadFileSignal,
                   [&](OAIHttpFileElement summary)
       {
            qDebug() << "Downloaded the file " << summary.local_filename;
       });

    QObject::connect(&api, &OpenAPI::OAIDefaultApi::upLoadFileSignal,
                   []()
       {
            qDebug() << "Uploaded the file ";
       });

    QObject::connect(&api, &OpenAPI::OAIDefaultApi::upLoadFileWithInfoSignal,
                   []()
       {
            qDebug() << "Uploaded the file with info";
       });
    QTimer::singleShot(0, [&]()
        {
            api.downLoadFile(QString("appsettings.json"));
            api.upLoadFile(fileId, fileElement);
            api.upLoadFileWithInfo(fileId, orderId, userId, fileElement);
        }
    );
    a.exec();
    return 0;
}

sample spec for test

openapi: 3.0.0
servers:
  - url: 'https://api.testapp.com'
info:
  description: Test API for download and upload features.
  version: 1.0.0
  title: testapp.com
paths:
  /downLoadFile/{fileId}:
    get:
      operationId: downLoadFile
      parameters:
        - name: fileId
          in: path
          schema:
            type: string
          required: true
      responses:
        '200':
          description: A picture file
          content:
            image/png:
              schema:
                type: string
                format: binary

  /upLoadFile/{fileId}:
    post:
      operationId: upLoadFile
      parameters:
        - name: fileId
          in: path
          schema:
            type: string
          required: true
      requestBody:
        content:
          image/png:
            schema:
              type: string
              format: binary          
      responses:
        '200':
          description: success
                
  /upLoadFileWithInfo/{fileId}:
    post:
      operationId: upLoadFileWithInfo
      parameters:
        - name: fileId
          in: path
          schema:
            type: string
          required: true
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                orderId:
                  type: integer
                userId:
                  type: integer
                fileName:
                  type: string
                  format: binary          
      responses:
        '200':
          description: success

cc: @stkrwork @MartinDelille @muttleyxd @ravinikam