[sap-document-ai]: Declarative Approach - Annotation based triggering

[samyuktaprabhu]

Currently, document extraction can only be triggered programmatically by emitting a DocumentExtraction event from application code.

Add support for declarative triggering via a CDS annotation (e.g. @DocumentAI) so that extraction is triggered automatically when an annotated entity field is written, without requiring boilerplate event emission code.

This should cover:

• Backend: plugin reacts to annotated field writes
• Fiori Elements UI: an "Upload & Extract" button injected automatically on the Object Page

[github-actions]

👋 Hello @samyuktaprabhu, thank you for submitting this issue. Our team is reviewing your report and will follow up with you as soon as possible.

[lisajulia]

Closing since document-ai part will move to own repo in https://github.com/cap-java.