Implement DynamoDbFlatten annotation

Description

Introduce the [DynamoDbFlatten] attribute, enabling developers to flatten complex nested types into their parent objects in DynamoDB, mirroring the behavior of Java SDK’s @DynamoDbFlattenDynamoDB Object Mapping. This allows for simpler, flat table schemas while maintaining clean object models in code.

Java DynamoDbFlatten

 [DynamoDBTable("HashTable")]
 public class EmployeeNonFlat
 {
     [DynamoDBHashKey("Id")]
     public int EmployeeId { get; set; }

     [DynamoDBFlatten]
     public ContactInfo Contact { get; set; }
 }

 public class ContactInfo
 {
     public string Email { get; set; }

     [DynamoDBFlatten]
     public Address Address { get; set; }
 }

Motivation and Context

Testing

Integration test added
Unit test to be added for internal methods

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license