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.Rest/API/Common/Message.cs

@@ -50,6 +50,8 @@ namespace Discord.API
         // sent with Rich Presence-related chat embeds
         [JsonProperty("application")]
         public Optional<MessageApplication> Application { get; set; }
+        [JsonProperty("message_reference")]
+        public Optional<MessageReference> Reference { get; set; }
         [JsonProperty("flags")]
         public Optional<MessageFlags> Flags { get; set; }
     }