Introduce Event Model for Offers Flow

[shaavan]

This PR introduces the event-model for Flow, that allows a user to asynchronously examine, and handle and InvoiceRequest, or an Invoice, based on their selected FlowConfig.# Introduce Event Model for Offers Flow

This PR introduces an event-based model for OffersMessageFlow, enabling users to asynchronously examine and handle incoming InvoiceRequests or Bolt12Invoices based on their configured FlowConfigs.

By setting custom FlowConfigs, users can choose whether to:

• Automatically respond to offers synchronously (as before), or
• Receive structured OfferEvents for manual inspection and handling.

This makes offer handling more flexible, especially in use-cases like:

• Dynamic conversion for currency-denominated offers
• Selective acceptance of invoices based on custom user-based rules

The default configuration (NeverTrigger) maintains backward compatibility by preserving the existing synchronous behavior.

[ldk-reviews-bot]

I've assigned @joostjager as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[shaavan]

cc @jkczyz