Add lazy implementation

Author: verdie-g

src/System.Linq.Dynamic.Core/Compatibility/Lazy.cs

@@ -0,0 +1,40 @@
+using System.Threading;
+
+#if NET35
+namespace System
+{
+    internal class Lazy<T>
+    {
+        private readonly Func<T> _valueFactory;
+        private readonly object _lock;
+
+        private T? _value;
+        private bool _valueCreated;
+
+        public Lazy(Func<T> valueFactory, LazyThreadSafetyMode mode)
+        {
+            _valueFactory = valueFactory;
+            _lock = new object();
+        }
+
+        public T Value
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    if (_valueCreated)
+                    {
+                        return _value!;
+                    }
+
+                    _value = _valueFactory();
+                    _valueCreated = true;
+                    return _value;
+                }
+            }
+        }
+
+    }
+}
+#endif

src/System.Linq.Dynamic.Core/Compatibility/LazyThreadSafetyMode.cs

@@ -0,0 +1,11 @@
+#if NET35
+namespace System.Threading
+{
+    internal enum LazyThreadSafetyMode
+    {
+        None,
+        PublicationOnly,
+        ExecutionAndPublication,
+    }
+}
+#endif