Maybe support "extra attributes" map?

Not sure if its possible but ... something like below for example:

```kotlin
@Serializable
class From(
  val foo: String,
  @ExtraAttributes val additionalAttributes: Map<String, AttributeValue>,
)

Dynamap.decodeFromItem<From>(mapOf(
  "foo" to S("Hello"),
  "bar" to S("World!"),
))

// Gives From(foo=Hello, additionalAttributes=(bar=World!))
```