Mapping AWS built-in scalar types

**Question**

How does one map an AWS built-in scalar type? Currently, using AWS scalar types results in generated models with "stringly typed" fields, which are bad practise and undesirable for a variety of well understood reasons.

If the schema defines:

```graphql
type Todo {
id: ID!
name: String!
description: String
created: AWSDateTime
}
```

How can we configure the code generator to output `created` with a date/time type of our choosing? Eg `ZonedDateTime`?

Since Apollo provides support for this functionality, I'd expect it to be available to AWS AppSync SDK users as well.

**Environment**

AppSync SDK `2.7.0`
Kotlin `1.3.10`
Android Studio 3.3 RC 1
Build #AI-182.5107.16.33.5138683, built on November 19, 2018
JRE: 1.8.0_152-release-1248-b01 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
macOS 10.13.6

**Sample project**

Attached is a sample project demonstrating how I'd expect this to work, based on the Apollo documentation:

[AppSyncCustomTypes.zip](https://github.com/awslabs/aws-mobile-appsync-sdk-android/files/2646696/AppSyncCustomTypes.zip)

It does not compile, because the generated type is always a `String`.

Start with the attached AppSyncCustomTypes.zip sample project and the generated model for the AWSDateTime field. Trace the code-generation configuration that turns AWS scalar types into String fields. Done means the generator supports a configurable date/time type such as ZonedDateTime and the sample project compiles with the generated field using that type.

Written by the indexing model from the issue text.