[Haskell] Add Scotty server generator

[wing328]

Description

As discussed in #6147, we want to add a Scotty server generator.

Ideally we want to create an abstract base class "AbstractHaskellCodegen.java" based on HaskellServantCodegen.java to start with so as to make all Haskell generators (client, server) more consistent and easier to maintain.

Swagger-codegen version

Latest master

Suggest a fix/enhancement

If anyone wants to contribute the new generator, please reply to let us know.

cc @kamyar1979 @algas @mandrean

[jonschoning]

I am working on a http://hackage.haskell.org/package/http-client client generator.
This will be targeted specifically at being a client.

I was simply creating a HaskellHttpClientCodegen.java for this, but using a AbstractHaskellCodegen.java could work too.

[wing328]

@jonschoning thanks! We can create the abstract class later. Let us know if you need any help.