Support for flattening objects

The official DynamoDB mapper (at least the 2.x one) has a `DynamoDbFlatten` that instructs it to flatten the specified property out into a single row:
```kotlin
@DynamoDbBean
data class BookItem(
@get:DynamoDbPartitionKey
var iban: String?
var name: String? = null,
@get:DynamoDbFlatten
var author: Author? = null
) {
@get:DynamoDbSortKey
var sortKey: String = "book"

@DynamoDbBean
data class Author(
@get:DynamoDbAttribute("authorName")
var name: String? = null,
@get:DynamoDbAttribute("authorNationality")
var nationality: String? = null
)
}
```

It would be nice if Tempest provided either an automatic or manual mapping of these nested types.

The issue does not name repository files, tests, or a mapper entry point; begin by locating Tempest's DynamoDB mapping code and comparing it with the AWS SDK's DynamoDbFlatten annotation described here. Define whether automatic or manual nested-type mapping is in scope, then verify that the BookItem and Author example can be represented as flattened attributes.

Written by the indexing model from the issue text.