add respondwithmodal methods to restinteractinmodulebase (#2227)

src/Discord.Net.Interactions/Extensions/RestExtensions.cs

@@ -0,0 +1,25 @@
+using Discord.Interactions;
+using System;
+
+namespace Discord.Rest
+{
+    public static class RestExtensions
+    {
+        /// <summary>
+        ///     Respond to an interaction with a <see cref="IModal"/>.
+        /// </summary>
+        /// <typeparam name="T">Type of the <see cref="IModal"/> implementation.</typeparam>
+        /// <param name="interaction">The interaction to respond to.</param>
+        /// <param name="options">The request options for this <see langword="async"/> request.</param>
+        /// <returns>Serialized payload to be used to create a HTTP response.</returns>
+        public static string RespondWithModal<T>(this RestInteraction interaction, string customId, RequestOptions options = null, Action<ModalBuilder> modifyModal = null)
+            where T : class, IModal
+        {
+            if (!ModalUtils.TryGet<T>(out var modalInfo))
+                throw new ArgumentException($"{typeof(T).FullName} isn't referenced by any registered Modal Interaction Command and doesn't have a cached {typeof(ModalInfo)}");
+
+            var modal = modalInfo.ToModal(customId, modifyModal);
+            return interaction.RespondWithModal(modal, options);
+        }
+    }
+}