Various updates.

Harmonia.Core/Caching/MemoryCache.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Options;
 
 namespace Harmonia.Core.Caching;
 
@@ -27,10 +28,16 @@ public abstract class MemoryCache<TKey, TValue> : Cache<TKey, TValue> where TKey
 
         var cacheEntryOptions = new MemoryCacheEntryOptions()
             .SetSize(entrySize)
-            .SetSlidingExpiration(SlidingExpiration);
+            .SetSlidingExpiration(SlidingExpiration)
+            .RegisterPostEvictionCallback(PostEvictionCallback);
 
         _memoryCache.Set(key, entry, cacheEntryOptions);
     }
 
+    protected virtual void PostEvictionCallback(object? cacheKey, object? cacheValue, EvictionReason evictionReason, object? state)
+    {
+
+    }
+
     protected abstract long GetEntrySize(TValue entry);
 }