feat: add id prop handling to default doc builder

[zyc]

✨ Proposal

This PR adds support for the id property in the default_doc_builder. Although optional, the id is a highly relevant field for many use cases, including unique document identification, tracking, and integration with external systems.

✅ Expected Behavior

• If an id is retrieved, it will be used by the default_doc_builder.
• If the id is absent at the time of index creation, the behavior remains unchanged — the system will continue to function exactly as it did before.

🛠️ Motivation

Supporting a pre-filled id reduces the need for custom logic elsewhere in the application and enables more consistent document handling when a specific identifier is required.

🔄 Compatibility

This is a non-breaking change. The id remains optional, ensuring full backward compatibility with existing implementations.