Add support for keyed service with multiple attributes

[nuzolx]

Hi,

I'd like to be able to specify the attribute several times in the case of keyed service. Is this possible? At the moment, error ‘CS0579’ is being thrown.

public interface INotificationService
{
    string Notify(string message);
}

[Service<string>("sms")]
public class SmsNotificationService : INotificationService
{
    public string Notify(string message) => $"[SMS] {message}";
}

[Service<string>("email")]
[Service<string>("default")]
public class EmailNotificationService : INotificationService
{
    public string Notify(string message) => $"[Email] {message}";
}