[Android] Document how to map AppSync Scalars to Kotlin types (#8205)

* document how to map appsync scalars to kotlin types

Author: tylerjroach

src/pages/[platform]/build-a-backend/data/aws-appsync-apollo-extensions/index.mdx

@@ -320,6 +320,19 @@ You can alternatively download the introspection schema using the [`fetch-schema
 2. On the left side, select Schema
 3. When viewing your schema, there should a “Export schema” drop down. Select this and download the `schema.json` file.
 4. Add this file to your project as directed by [Apollo documentation](https://www.apollographql.com/docs/kotlin/advanced/plugin-recipes#specifying-the-schema-location) 
+
+### Type Mapping AppSync Scalars
+By default, [AWS AppSync Scalars](https://docs.aws.amazon.com/appsync/latest/devguide/scalars.html#graph-ql-aws-appsync-scalars) will default to the `Any` type. You can map these scalars to more explicit types by editing the `apollo` block in your `app/build.gradle[.kts]` file. In the example below, we are now mapping a few of our AppSync scalar types to `String` instead of `Any`. Additional improvements could be made by writing [custom class adapters](https://www.apollographql.com/docs/kotlin/essentials/custom-scalars#define-class-mapping) to convert date/time scalars into Kotlin date/time class types.
+
+```kotlin
+apollo {
+    service("{serviceName}") {
+        packageName.set("{packageName}")
+        mapScalarToKotlinString("AWSDateTime")
+        mapScalarToKotlinString("AWSEmail")
+    }
+}
+```
 </InlineFilter>
 
 ### Performing Queries, Mutations, and Subscriptions with Apollo client