Improved hidden service handling and event management

[yallie]

1. Hidden system services shouldn't be remotely callable:

var proxy = client.CreateProxy<IDelegateProxyFactory>();
proxy.Create(typeof(Action), null)); // throws RemoteInvocationException: service is hidden

2. IDependencyInjectionContainer should be resolvable:

public class MyCustomService(IDependencyInjectionContainer container)
{
     // container can be used to resolve other services on demand
}

3. Added a hidden server-side service for invoking events:

// server component can relay other service's events
eventDispatcher.Invoke<ITestService>(s => nameof(s.ServiceEvent), EventArgs.Empty);

4. Added reverse index to ClientDelegateRegistry for faster lookup by delegate + proxy pair:

var indexKey = (clientDelegate, serviceProxy); // key is allocated on stack, not heap

if (_reverseIndex.TryGetValue(indexKey, out var existing)) // O(1) lookup
    return existing;

5. Implemented event subscription tracker, to compare active subscriptions on client and server:

var proxy = client.CreateProxy<ITestService>();
proxy.ServiceEvent += eventHandler;

Assert.Equal(1, client.SubscriptionTracker.Count); // SubscriptionTracker is internal, not a public API
Assert.Equal(client.SubscriptionTracker.Checksum, serverTracker.Checksum);