Support mapping custom scalars to generic classes with type parameters.

[martinbonnin]

Right now, we can map custom scalars to Kotlin classes using a Gradle configuration:

apollo {
    customTypeMapping.set(mapOf(
    "Date" to "java.util.Date"
  ))
}

This doesn't work anymore with generic types:

apollo {
    customTypeMapping.set(mapOf(
    "Json" to "kotlin.collection.Map<String, Any?>"
  ))
}

One simple workaround is to use a type alias:

typealias JsonScalar = Map<String, Any?>

But that's adding a level of indirection. Is there value in adding generics support?

[BoD]

In #6158 we now accept the simple cases:

• com.example.Type / com.example.Type?
• com.example.Type<com.example.Type1, com.example.Type2>
• common Kotlin and Java types like String, Map, etc. don't need to include the package name

More complex cases like Type<Type1<Type2>>, Type<*> or Type<out Type1> aren't covered. They would need a more advanced Kotlin/Java parser (maybe that could make a nice contribution to KotlinPoet/JavaPoet?).