submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Message/Entity/CommonResolver.cs

@@ -0,0 +1,34 @@
+namespace Lagrange.OneBot.Message.Entity;
+
+public static class CommonResolver
+{
+    private static readonly HttpClient Client = new();
+
+    public static byte[]? Resolve(string url)
+    {
+        if (url.StartsWith("base64://")) return Convert.FromBase64String(url.Replace("base64://", ""));
+
+        Uri uri = new(url);
+
+        return uri.Scheme switch
+        {
+            "http" or "https" => Client.GetAsync(uri).Result.Content.ReadAsByteArrayAsync().Result,
+            "file" => File.ReadAllBytes(Path.GetFullPath(uri.LocalPath)),
+            _ => null,
+        };
+    }
+
+    public static Stream? ResolveStream(string url)
+    {
+        if (url.StartsWith("base64://")) return new MemoryStream(Convert.FromBase64String(url.Replace("base64://", "")));
+
+        Uri uri = new(url);
+
+        return uri.Scheme switch
+        {
+            "http" or "https" => Client.GetAsync(uri).Result.Content.ReadAsStreamAsync().Result,
+            "file" => new FileStream(Path.GetFullPath(uri.LocalPath), FileMode.Open, FileAccess.Read, FileShare.Read),
+            _ => null,
+        };
+    }
+}