Added SZ singleton and services

SzCore/Defaults/DefaultLocalFileManager.cs

@@ -0,0 +1,51 @@
+namespace SzCore.Defaults;
+
+public class DefaultLocalFileManager : ISzFileManager
+{
+    public string DataPath
+    {
+        get
+        {
+            var path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "data");
+            if (!Directory.Exists(path)) 
+            {
+                Directory.CreateDirectory(path);
+            }
+            
+            return path;
+        }
+    }
+
+    public string DatasetsPath => Path.Combine(DataPath, "datasets");
+
+    public bool SaveFile(string path, string fileContent)
+    {
+        try
+        {
+            var dir = Path.GetDirectoryName(path);
+            if (dir is null) return false;
+            Directory.CreateDirectory(dir);
+            
+            File.WriteAllText(path, fileContent);
+            return true;
+        }
+        catch (Exception e)
+        {
+            Console.WriteLine($"Error saving file: {e.Message}");
+            return false;
+        }
+    }
+
+    public string? LoadFile(string path)
+    {
+        try
+        {
+            return File.ReadAllText(path);
+        }
+        catch (Exception e)
+        {
+            Console.WriteLine($"Error loading file: {e.Message}");
+            return null; 
+        }
+    }
+}