[sap-document-ai]: Support SAP Document AI templates #116

The plugin currently performs document extraction without exposing SAP Document AI template capabilities. Applications cannot associate documents with templates or select a template when initiating extraction.

Add support for SAP Document AI templates so applications can leverage template-based extraction for both standard and custom document types. This could include:

• Selecting a template when triggering document extraction.
• Configuring a default template via annotations or application.yaml.
• Allowing applications to override the template at runtime when needed.

This would enable higher extraction accuracy for documents with known layouts while making it easier to process custom document types supported by SAP Document AI.