simplify PluginManager a bit more

Pilz.Plugins/PluginRuntimeInfo{T}.cs

@@ -0,0 +1,10 @@
+namespace Pilz.Plugins;
+
+public class PluginRuntimeInfo<T> : PluginRuntimeInfo where T : class
+{
+    public new T? Plugin
+    {
+        get => base.Plugin as T;
+        set => base.Plugin = value;
+    }
+}