feat: add CacheItemPolicy for backwards compat

LazyCache/CacheItemPolicy.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Linq;
+using Microsoft.Extensions.Caching.Memory;
+
+namespace LazyCache
+{
+    [Obsolete(
+        "CacheItemPolicy was part of System.Runtime.Caching which is no longer used by LazyCache. " +
+        "This class is a fake used to maintain backward compatibility and will be removed in a later version."+
+        "Change to MemoryCacheEntryOptions instead")]
+    public class CacheItemPolicy : MemoryCacheEntryOptions
+    {
+        public PostEvictionCallbackRegistration RemovedCallback => PostEvictionCallbacks.FirstOrDefault();
+    }
+}