Fix method rpc registration

Reactor/Networking/CustomRpcManager.cs

@@ -47,7 +47,10 @@ namespace Reactor.Networking
             _list.Add(customRpc);
             _map[customRpc.InnerNetObjectType].Add(customRpc);
 
-            typeof(Rpc<>).MakeGenericType(customRpc.GetType()).GetProperty("Instance")!.SetValue(null, customRpc);
+            if (customRpc.IsSingleton)
+            {
+                typeof(Rpc<>).MakeGenericType(customRpc.GetType()).GetProperty("Instance")!.SetValue(null, customRpc);
+            }
 
             return customRpc;
         }

Reactor/Networking/MethodRpc/MethodRpc.cs

@@ -51,6 +51,8 @@ namespace Reactor.Networking.MethodRpc
         public MethodInfo Method { get; }
         public HandleDelegate Handle { get; }
 
+        protected internal override bool IsSingleton => false;
+
         public override Type InnerNetObjectType { get; }
         public override RpcLocalHandling LocalHandling { get; }
         public override SendOption SendOption { get; }

Reactor/Networking/UnsafeCustomRpc.cs

@@ -9,6 +9,7 @@ namespace Reactor.Networking
     public abstract class UnsafeCustomRpc
     {
         internal CustomRpcManager? Manager { get; set; }
+        protected internal virtual bool IsSingleton => true;
 
         public uint Id { get; }
         public BasePlugin UnsafePlugin { get; }