Add new feature.

Added ILogger;
Added IGraphProcessor;

Changed return type of OnExecute method in BaseNode from void to bool;

Sample/Runtime/Models/Logger.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+
+namespace Misaki.GraphView.Sample
+{
+    public class Logger : ILogger
+    {
+        private readonly List<string> _logs = new ();
+        
+        public Action<BaseNode, string, LogType> OnLog { get; set; }
+        
+        public void LogInfo(BaseNode node, string message)
+        {
+            _logs.Add($"Log Info from node {node.GetType().Name}: {message}");
+            OnLog?.Invoke(node, message, LogType.Info);
+        }
+
+        public void LogWarning(BaseNode node, string message)
+        {
+            _logs.Add($"Log Warning from node {node.GetType().Name}: {message}");
+            OnLog?.Invoke(node, message, LogType.Warning);
+        }
+
+        public void LogError(BaseNode node, string message)
+        {
+            _logs.Add($"Log Error from node {node.GetType().Name}: {message}");
+            OnLog?.Invoke(node, message, LogType.Error);
+        }
+    }
+}