[Haskell] Refactor generator to create separate client, server generator

Description

Currently, Haskell generator (HaskellServantCodegen.java) generates both client and server code. We want to refactor HaskellServantCodegen.java into the following:

HaskellServantCodegen.java - for server HaskellClientCodegen.java - for client AbstractHaskellCodegen.java - abstract class for client/server generator

The goal is to make Haskell generator more consistent with other generators (as no other generator output both client and server code at the same time)

Swagger-codegen version

Latest master

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