chore: Allow custom image resizer to be an async method

lib/src/client.dart

@@ -104,8 +104,8 @@ class Client extends MatrixApi {
 
   final Duration sendTimelineEventTimeout;
 
-  MatrixImageFileResizedResponse? Function(MatrixImageFileResizeArguments)?
-      customImageResizer;
+  Future<MatrixImageFileResizedResponse?> Function(
+      MatrixImageFileResizeArguments)? customImageResizer;
 
   /// Create a client
   /// [clientName] = unique identifier of this client

lib/src/utils/matrix_file.dart

@@ -105,7 +105,8 @@ class MatrixImageFile extends MatrixFile {
       required String name,
       int maxDimension = 1600,
       String? mimeType,
-      MatrixImageFileResizedResponse? Function(MatrixImageFileResizeArguments)?
+      Future<MatrixImageFileResizedResponse?> Function(
+              MatrixImageFileResizeArguments)?
           customImageResizer,
       Future<T> Function<T, U>(FutureOr<T> Function(U arg) function, U arg)?
           compute}) async {
@@ -115,8 +116,9 @@ class MatrixImageFile extends MatrixFile {
       fileName: name,
       calcBlurhash: true,
     );
-    customImageResizer ??= _resize;
-    final resizedData = compute != null
+    final resizedData = customImageResizer != null
+        ? await customImageResizer(arguments)
+        : compute != null
             ? await compute(_resize, arguments)
             : _resize(arguments);
 
@@ -157,7 +159,8 @@ class MatrixImageFile extends MatrixFile {
   /// computes a thumbnail for the image
   Future<MatrixImageFile?> generateThumbnail(
       {int dimension = Client.defaultThumbnailSize,
-      MatrixImageFileResizedResponse? Function(MatrixImageFileResizeArguments)?
+      Future<MatrixImageFileResizedResponse?> Function(
+              MatrixImageFileResizeArguments)?
           customImageResizer,
       Future<T> Function<T, U>(FutureOr<T> Function(U arg) function, U arg)?
           compute}) async {