feature: Implemented Message Reference Property (#1413)

* Added support for Message References

* Removed unused usings, added debugger display, updated ToString override

* Changed snowflakes to be wrapped in an optional instead of a nullable.

src/Discord.Net.Core/Entities/Messages/IMessage.cs

@@ -140,6 +140,18 @@ namespace Discord
         /// </returns>
         MessageApplication Application { get; }
 
+        /// <summary>
+        ///     Gets the reference to the original message if it was crossposted.
+        /// </summary>
+        /// <remarks>
+        ///     Sent with Cross-posted messages, meaning they were published from news channels
+        ///     and received by subscriber channels.
+        /// </remarks>
+        /// <returns>
+        ///     A message's reference, if any is associated.
+        /// </returns>
+        MessageReference Reference { get; }
+
         /// <summary>
         ///     Gets all reactions included in this message.
         /// </summary>

src/Discord.Net.Core/Entities/Messages/MessageReference.cs

@@ -0,0 +1,33 @@
+using System.Diagnostics;
+
+namespace Discord
+{
+    /// <summary>
+    ///     Contains the IDs sent from a crossposted message.
+    /// </summary>
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class MessageReference
+    {
+        /// <summary>
+        ///     Gets the Message ID of the original message.
+        /// </summary>
+        public Optional<ulong> MessageId { get; internal set; }
+
+        /// <summary>
+        ///     Gets the Channel ID of the original message.
+        /// </summary>
+        public ulong ChannelId { get; internal set; }
+
+        /// <summary>
+        ///     Gets the Guild ID of the original message.
+        /// </summary>
+        public Optional<ulong> GuildId { get; internal set; }
+
+        private string DebuggerDisplay
+            => $"Channel ID: ({ChannelId}){(GuildId.IsSpecified ? $", Guild ID: ({GuildId.Value})" : "")}" +
+            $"{(MessageId.IsSpecified ? $", Message ID: ({MessageId.Value})" : "")}";
+
+        public override string ToString()
+            => DebuggerDisplay;
+    }
+}