[SWIFT] Alamofire Serialization on another queue than main

Description

There is a problem in the AlamofireDecodableRequestBuilder class or maybe rather a lack of way to make serialization of responses run on another queue than the main queue.

In the processRequest method there is multiple cases on how to handle the response based on the data type. In Alamofire it is possible to pass a queue to run the serialization on. With large amounts of data this is causing the main thread to be blocked (locking up the user interface).

Docs on how to do it in Alamofire:

https://github.com/Alamofire/Alamofire/blob/master/Documentation/AdvancedUsage.md#response-mapping

Swagger-codegen version

2.3.1

Command line used for generation

java -jar $jar_codegen_file generate -i $json_spec_file -o . -l swift4 --additional-properties responseAs=RxSwift --additional-properties unwrapRequired=true

Steps to reproduce

1. Generate Swift code from Swagger spec.
2. Serialize big amounts of data
3. Main thread is being blocked

Suggest a fix/enhancement

Make it possible to generate AlamofireDecodableRequestBuilder with a specific queue which the serialization should happen on.

Start with the generated Swift AlamofireDecodableRequestBuilder and its processRequest method, then compare the requested behavior with Alamofire's response-mapping documentation. The change is done when generated clients can specify a queue for response serialization so large responses do not block the main thread; no test file is named in the issue.