DynamoDB Enhanced Client Polymorphic Types

We should support reading/writing base types using the DynamoDB Enhanced Client.

Potential syntax/example (not yet implemented):

```java
@DynamoDbBean
@DynamoDbSubtypes({Employee.class, Customer.class})
public abstract class Person {
@DynamoDBHashKey
private long id;
private String name;
}

@DynamoDbBean
private class Employee extends Person {
}

@DynamoDbBean
private class Customer extends Person {
}

DynamoDbEnhancedClient client = DynamoDbEnhancedClient.create();
DynamoDbTable people = client.table("people", TableSchema.fromBean(Person.class));

Employee bob = new Employee();
bob.setId(1);
bob.setName("Bob the Builder");

Customer lfh = new Customer();
lfh.setId(2);
lfh.setName("Low Flying Hawk");

people.putItem(bob);
people.putItem(lfh);

assertThat(people.getItem(Key.builder().partitionValue(1).build())).isInstanceOf(Employee.class);
assertThat(people.getItem(Key.builder().partitionValue(2).build())).isInstanceOf(Customer.class);
```

The issue names DynamoDbEnhancedClient.table, TableSchema.fromBean, and DynamoDbTable but no repository files or tests. Start by tracing how TableSchema.fromBean maps the base type, then design coverage for the Employee and Customer round trip shown. Done means base-typed reads return the correct concrete subtype and both subtype writes remain readable.

Scritto dal modello di indicizzazione a partire dal testo della issue.