Integrate CloudEvents into the abstraction library

[Copilot]

This PR integrates CloudEvents support into the OAgents abstraction library while maintaining full backward compatibility with existing Event-based code.

Changes Made

Enhanced Event Class

The Event class now wraps CloudNative.CloudEvents.CloudEvent internally while preserving the existing API:

// Traditional usage still works
var evt = new Event
{
    Type = "user.action",
    Subject = "user-123", 
    Data = new Dictionary<string, string> { { "action", "login" } }
};

// New CloudEvent usage
var cloudEvent = new CloudEvent
{
    Type = "com.example.user.login",
    Source = new Uri("https://example.com/users"),
    Id = Guid.NewGuid().ToString(),
    Subject = "user-123",
    Data = new Dictionary<string, string> { { "action", "login" } }
};

// Seamless conversion between types
Event convertedEvent = cloudEvent; // Implicit conversion
CloudEvent backToCloudEvent = convertedEvent.CloudEvent;

Extended IAgent Interface

Added CloudEvent overloads alongside existing Event methods:

public interface IAgent
{
    // Existing Event methods (unchanged)
    Task HandleEvent(Event item);
    Task PublishEvent(string ns, string id, Event item);
    
    // New CloudEvent methods
    Task HandleEvent(CloudEvent item);
    Task PublishEvent(string ns, string id, CloudEvent item);
}

Updated Base Implementations

• Orleans Agent: Supports CloudEvent with automatic conversion to Event for stream processing
• Dapr Agent: Publishes CloudEvents directly with proper metadata for Dapr pub/sub

Package Dependencies

• Added CloudNative.CloudEvents to Core, Orleans, and Dapr projects
• Uses CloudEvents v2.8.0 for latest standards compliance

Benefits

1. Standards Compliance: Follows CNCF CloudEvents specification
2. Interoperability: Works with other CloudEvents-compliant systems
3. Rich Metadata: Supports Source, Time, Id, and other CloudEvent properties
4. Future-Proof: Aligns with industry event standards
5. Zero Breaking Changes: All existing code continues to work unchanged

Validation

• All sample applications build successfully
• Integration tests verify seamless conversion between Event and CloudEvent
• Backward compatibility maintained across Orleans and Dapr implementations

Fixes #48.