Add support for DynamoDBAutoGeneratedTimestampAttribute that sets current timestamp during persistence operations.

[irina-herciu]

Add support for the [DynamoDBAutoGeneratedTimestamp] attribute to the .NET SDK for DynamoDB, enabling automatic setting of timestamp fields during save operations. This feature aligns with the Java SDK's @DynamoDbAutoGeneratedTimestampAttribute.

Description

• New Attribute: [DynamoDBAutoGeneratedTimestamp]

  • Automatically sets the property to the current UTC timestamp (DateTime.UtcNow) on item save.

• Modes:

  • Create: Set timestamp only when the item is first created.
  • Always: Set timestamp on both create and update.

• Usage:

  class Order
  {
      [DynamoDBHashKey]
      public string Id { get; set; }
  
      [DynamoDBAutoGeneratedTimestamp(TimestampMode.Create)]
      public DateTime CreatedAt { get; set; }
  }

Motivation and Context

Testing

unit and integration tests added

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