submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Core/Entity/OneBotEntityBase.cs

@@ -0,0 +1,18 @@
+using System.Text.Json.Serialization;
+
+namespace Lagrange.OneBot.Core.Entity;
+
+[Serializable]
+public abstract class OneBotEntityBase(uint selfId, string postType, long time)
+{
+    public OneBotEntityBase(uint selfId, string postType)
+        : this(selfId, postType, DateTimeOffset.Now.ToUnixTimeSeconds())
+    {
+    }
+
+    [JsonPropertyName("time")] public long Time { get; set; } = time;
+
+    [JsonPropertyName("self_id")] public uint SelfId { get; set; } = selfId;
+
+    [JsonPropertyName("post_type")] public string PostType { get; set; } = postType;
+}