[haskell-http-client] add support for import-mappings

[jonschoning]

import mappings take the form of:

--import-mappings %DATATYPE%=%MODULE%

full example (with --type-mappings):

--type-mappings intstr.IntOrString=IntOrString
--import-mappings IntOrString=Kubernetes.CustomTypes

The import-mapped module will be an exposed module of the generated library.

The import mappings will be re-exported from ImportMappings, which itself is re-xported from Models.

Model.hs

module Kubernetes.Model (module Kubernetes.Model, module Kubernetes.ImportMappings) where
import Kubernetes.ImportMappings

ImportMappings.hs:

module Kubernetes.ImportMappings (module ImportMappings) where
import Kubernetes.CustomTypes as ImportMappings (IntOrString(..))

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[jonschoning]

cc @guoshimin

[jonschoning]

ref issue #2362

[guoshimin]

We also need to have a module in the test directory to define the Arbitrary instances for the custom types.