Add MultiSelectTemplate

Author: Yanan Wang

src/Infrastructure/BotSharp.Abstraction/Messaging/Models/RichContent/Template/MultiSelectTemplate.cs

@@ -0,0 +1,17 @@
+
+namespace BotSharp.Abstraction.Messaging.Models.RichContent.Template
+{
+    public class MultiSelectTemplate : TextMessage
+    {
+        [JsonPropertyName("template_type")]
+        public string TemplateType => "multi-select";
+        public List<OptionElement> Options { get; set; } = new List<OptionElement>();
+    }
+
+    public class OptionElement
+    {
+        public string Title { get; set; } = string.Empty;
+        public string Type { get; set; } = string.Empty;
+        public string? Payload { get; set; }
+    }
+}