[Feature]: Configurable Handler Lifetime

[eneshoxha]

Title: Allow configuring handler service lifetime (Scoped, Transient, Singleton)

Labels: enhancement, mediator

Body:

Problem

In ServiceCollectionExtensions.cs, all handlers (ICommandHandler, IQueryHandler, INotificationHandler, IStreamQueryHandler) are registered with WithScopedLifetime() hardcoded. There is no way to configure this.

Some architectures prefer:

• Transient handlers when handlers are stateless and scope leaks are a concern
• Singleton handlers for high-throughput scenarios with stateless handlers

Proposed Solution

Add a HandlerLifetime property to MediatorOptions:

public class MediatorOptions
{
    public ServiceLifetime HandlerLifetime { get; set; } = ServiceLifetime.Scoped;
    // ...existing properties...
}

Use this in RegisterHandlers instead of hardcoded .WithScopedLifetime().