Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/Nadeko.Medusa/Attributes/svcAttribute.cs

@@ -0,0 +1,23 @@
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks the class as a service which can be used within the same Medusa
+/// </summary>
+[AttributeUsage(AttributeTargets.Class)]
+public class svcAttribute : Attribute
+{
+    public Lifetime Lifetime { get; }
+    public svcAttribute(Lifetime lifetime)
+    {
+        Lifetime = lifetime;
+    }
+}
+
+/// <summary>
+/// Lifetime for <see cref="svcAttribute"/>
+/// </summary>
+public enum Lifetime
+{
+    Singleton,
+    Transient
+}