Added IExecutable and ISlotContainer interface.

Changed SlotContainerNode to ExecutableNode

Runtime/Contracts/Nodes/IExecutable.cs

@@ -0,0 +1,15 @@
+namespace Misaki.GraphView
+{
+    public interface IExecutable
+    {
+        /// <summary>
+        /// Execute the node.
+        /// </summary>
+        public void Execute();
+        
+        /// <summary>
+        /// Clear the execution flag.
+        /// </summary>
+        public void ClearExecutionFlag();
+    }
+}