[sap-document-ai]: Add 'cds add document-ai' scaffold command #105

Setting up the Document AI plugin currently requires several manual configuration steps, including adding the required Maven dependency, enabling the persistent outbox in application.yaml, configuring the Document AI service binding, and creating boilerplate handler stubs.

These manual steps increase the effort required to get started and make onboarding more error-prone.

Introduce a cds add document-ai CLI command that automates the initial project setup.

The command should configure the required project components, including:

• Adding the required dependencies.
• Configuring the Document AI service binding (for example in mta.yaml).
• Enabling the persistent outbox.
• Generating the required handler stubs.

The goal is to reduce setup effort and provide a consistent onboarding experience for new adopters.