These types are used by the AsyncObjectMapper. Here's the magic which makes
object mapper schemas type safe and complete. You generally won't need to
make use of these types within your own code; just ObjectMapper should be
enough.
A function that takes some input object, and an optional context object, and returns a
promise of an output. This is used as part of an AsyncObjectMapperSchema.
A callable function, equivalent to calling AsyncObjectMapper#map.
It also exposes AsyncObjectMapperFunction#schema as a readonly property.
| AllowInputKeyIfInputCanExtendOutput<TInput, TOutput[TOutputKey]>
A mapper function, or input property name, used in an AsyncObjectMapperSchema.
An object, where every property name must match a property name in the desired output type.
Every property value must be a AsyncMapperSchemaValue.