[Feature]: Remove Scrutor Dependency / Offer Manual Registration

[eneshoxha]

Title: Reduce dependency footprint by making Scrutor optional and offering manual handler registration

Labels: enhancement, mediator

Body:

Problem

The core Cortex.Mediator package depends on Scrutor 6.0.1 for assembly scanning. This:

• Adds a transitive dependency that consumers may not want
• Prevents use in environments where assembly scanning is undesirable (e.g., Blazor WASM, NativeAOT)
• Is the only non-Microsoft dependency in the core package

Proposed Solution

Option A (minimal): Add a manual registration API alongside assembly scanning:

services.AddCortexMediator(options =>
{
    options.AddCommandHandler<CreateUserCommand, Guid, CreateUserCommandHandler>();
    options.AddQueryHandler<GetUserQuery, UserDto, GetUserQueryHandler>();
    options.AddNotificationHandler<UserCreatedNotification, SendEmailHandler>();
    // Pipeline behaviors as today
});

Option B (structural): Move Scrutor-based scanning to a separate Cortex.Mediator.Scrutor package:

• Core Cortex.Mediator has zero non-Microsoft dependencies
• Cortex.Mediator.Scrutor provides services.AddCortexMediator(markerTypes, ...) with assembly scanning

Both options can coexist and are not mutually exclusive.