[Feature]: Enhance GreekInvoicingToolkit with helpers for preset setup and etimologiera integration (AppointBuddy)

[PapLabros]

Describe the solution you'd like

Enhance the GreekInvoicingToolkit by introducing helper utilities and abstractions to simplify the setup of invoicing presets and their integration with the etimologiera provider.

Problem
The current setup process for configuring invoicing in the AppointBuddy application is complex and requires multiple manual steps:

1. Define payment methods in the Invoicing plugin
2. Map those payment methods to application payment methods
3. Create receipt presets and link them to text-based templates
4. Create invoice presets in the etimologiera plugin and link them back to the Invoicing plugin
5. Create payment type selectors that are used for mapping the payment methods of the Invoicing plug in to the payment types that are transmitted to MyData

This process is error-prone and not user-friendly, especially for users with limited technical knowledge.

Proposed solution

• Introduce helper utilities within GreekInvoicingToolkit to:
  • Simplify creation of receipt and invoice presets
  • Automatically link presets with etimologiera provider configurations
• Provide guided or automated setup flows (e.g., wizard or predefined templates)
• Create payment selector components to:
  • Map Invoicing plugin payment methods to MyData-compatible payment types
  • Simplify configuration and ensure correctness
• Reduce the number of manual steps required for initial setup

Alternatives you've considered

No response

Additional context

No response