Add customizable callbacks OnDllLoaded and Unloaded

Allows users to write their custom code in these callback functions.
OnDllLoaded, OnBeforeDllUnload, OnAfterDllUnload
Currently only the name is passed, this could be extended to be the NativeDll class (which is currently internal)

Author: rogerbarton

scripts/DllCallbacks.cs

@@ -0,0 +1,35 @@
+namespace UnityNativeTool
+{
+    /// <summary>
+    /// A place to implement you custom callbacks.
+    /// </summary>
+    public static class DllCallbacks
+    {
+        /// <summary>
+        /// This is called whenever a dll has been loaded.
+        /// </summary>
+        /// <param name="dllName">The name without preceding underscores or file extension.</param>
+        public static void OnDllLoaded(string dllName)
+        {
+            // Debug.Log("[dll] " + dllName + " dll loaded.");
+        }
+        
+        /// <summary>
+        /// This is called whenever a dll is about to be unloaded.
+        /// </summary>
+        /// <param name="dllName">The name without preceding underscores or file extension.</param>
+        public static void OnBeforeDllUnload(string dllName)
+        {
+            // Debug.Log("[dll] " + dllName + " dll unloading...");
+        }
+        
+        /// <summary>
+        /// This is called whenever a dll has been unloaded.
+        /// </summary>
+        /// <param name="dllName">The name without preceding underscores or file extension.</param>
+        public static void OnAfterDllUnload(string dllName)
+        {
+            // Debug.Log("[dll] " + dllName + " dll unloaded.");
+        }
+    }
+}

scripts/DllManipulator.cs

@@ -76,12 +76,14 @@ public static void UnloadAll()
                     if (dll.handle != IntPtr.Zero)
                     {
                         LowLevelPluginManager.OnBeforeDllUnload(dll);
+                        DllCallbacks.OnBeforeDllUnload(dll.name);
 
                         bool success = SysUnloadDll(dll.handle);
                         if (!success)
                             Debug.LogWarning($"Error while unloading DLL \"{dll.name}\" at path \"{dll.path}\"");
 
                         dll.ResetAsUnloaded();
+                        DllCallbacks.OnAfterDllUnload(dll.name);
                     }
                 }
             }
@@ -424,6 +426,7 @@ internal static void LoadTargetFunction(NativeFunction nativeFunction, bool igno
                 {
                     dll.loadingError = false;
                     LowLevelPluginManager.OnDllLoaded(dll);
+                    DllCallbacks.OnDllLoaded(dll.name);
                 }
             }