[Swift 4] Add ability to pass in a dedicated queue for processing network response

[joeboyscout04]

Description

The Swift generator uses Alamofire under the hood, which provides this ability to pass in a dedicated background queue for processing responses. The default is the Main thread. This can cause performance problems when the body of the request is large.

openapi-generator version

3.0.0

Suggest a fix/enhancement

It would be simple to alter the Swift generator's AlamofireImplementations execute(completion: error:) method to include an optional argument for queue (default to nil). The queue could then be passed to processRequest(completion:) and then on to the various Alamofire response() methods.

Then the user could implement their own AlamofireRequestBuilder to override the queue.