Add Akka.NET containers for SAGA management in event sourcing microservices

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This PR implements a comprehensive SAGA pattern solution using Akka.NET actors for managing distributed transactions in the microservice architecture, addressing the need for reliable event sourcing capabilities.

What's New

Core SAGA Infrastructure

• SagaCoordinatorActor: Manages individual SAGA lifecycles with state transitions and compensation logic
• EisenSagaActor: Handles business logic for Eisen (project/requirement) creation with a 3-step workflow
• SagaManagerActor: Coordinates multiple SAGA instances across the system
• Message System: Complete set of command/event messages for SAGA coordination

API Enhancements

The EisenController now supports SAGA-based operations:

# Create an Eisen with SAGA coordination
POST /api/eisen
{
  "Title": "Quantum Computer",
  "Description": "Build a universal quantum computer",
  "Phase": "Research ontwerp"
}

# Track SAGA progress
GET /api/eisen/saga/{sagaId}/status

# Monitor active SAGAs
GET /api/eisen/sagas/count

SAGA Workflow Example

For Eisen creation, the SAGA executes these compensatable steps:

1. Validation: Validates input data
2. Persistence: Saves to database (simulated)
3. Notification: Sends external notifications (simulated)

If any step fails, automatic compensation rolls back completed steps in reverse order.

Technical Details

Architecture Updates

• Upgraded from .NET Core 1.0 to .NET 8.0 for modern framework support
• Integrated Akka.NET 1.5.13 with cluster-ready configuration
• Implemented choreography-based SAGA pattern for loose coupling
• Added comprehensive error handling and compensation logic

Key Benefits

• Reliability: Akka.NET supervision strategies ensure fault tolerance
• Scalability: Actor-based design supports horizontal scaling
• Observability: Built-in logging shows SAGA execution flow
• Maintainability: Clear separation of concerns between SAGA steps

Configuration

The ActorSystem is configured in Startup.cs with proper dependency injection:

var actorSystem = ActorSystem.Create("SagaSystem", akkaConfig);
services.AddSingleton(actorSystem);
services.AddSingleton<ISagaService, SagaService>();

Testing

All endpoints have been tested and verified:

• SAGA creation and execution work correctly
• Status tracking provides real-time updates
• Actor message flow is properly logged
• Compensation logic is ready for failure scenarios

This implementation provides a solid foundation for distributed transaction management and event sourcing in the microservice tutorial, demonstrating how actor models can effectively manage complex business workflows.

Fixes #1.

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