[REQ] [Swift] Select response queue

[yosshi4486]

Is your feature request related to a problem? Please describe.

In swift4 APIClient, Main queue is used for response callback queue. In some cases, I want to use other queue.

Describe the solution you'd like

Pass queue to function argument that we want to use.

Describe alternatives you've considered

1. Add queue parameter to API like static var queue: DispatchQueue = .main in APIs.mustache
2. Add queue to response function argument in AlamofireImplementations.mustache

Developers can customize queue by assigning value to queue parameter. e.g. OpenAPI.queue = .global()

[yosshi4486]

similar to #2000. That pr changes default queue to background. but, I think user can select queue that they want. so I created new pr.