[R] code generator should support S4 classes

Description

Hi,
currently the templates for R code generation use R6Class to keep the object model:
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/r/model.mustache

#' @export
{{classname}} <- R6::R6Class(  '{{classname}}', public = list(...)

Projects like the bioconductor.org world prefer S4 classes among their packages,
and following that (strong) recommendation helps to maintain interoperability between
the packages.

So the enhancement would be to (also) have the possibility to create
the object hierarchy as S4 classes. @nilshoffmann has a project
that uses swagger and wants S4 classes to be emitted.

Swagger-codegen version

master is currently 2.4.4

Labels suggested are:
client:R and enhancement:feature or enhancement:code format.

Yours, Steffen

Start with modules/swagger-codegen/src/main/resources/r/model.mustache and compare the requested S4 object hierarchy with the current R6 output. Done means R code generation can emit S4 classes as an option while keeping the existing R6 generation available.