submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Utility/FallbackAsyncT.cs

@@ -0,0 +1,27 @@
+namespace Lagrange.OneBot.Utility.Fallbacks;
+
+public class FallbackAsync<T>
+{
+    private readonly List<Func<CancellationToken, Task<T?>>> _executors = [];
+
+    internal static FallbackAsync<T> Create()
+    {
+        return new();
+    }
+
+    public FallbackAsync<T> Add(Func<CancellationToken, Task<T?>> executor)
+    {
+        _executors.Add(executor);
+        return this;
+    }
+
+    public async Task<T> ExecuteAsync(Func<CancellationToken, Task<T>> @default, CancellationToken token = default)
+    {
+        foreach (var executor in _executors)
+        {
+            var result = await executor(token);
+            if (result != null) return result;
+        }
+        return await @default(token);
+    }
+}