feature: Add inline replies (#1659)

* Add inline replies

* Missed a few things

* Change xml docs, IUserMessage, and other changes

* Missed one when changing

* Fix referencedMessage author

src/Discord.Net.Rest/API/Common/Message.cs

@@ -56,5 +56,7 @@ namespace Discord.API
         public Optional<MessageFlags> Flags { get; set; }
         [JsonProperty("allowed_mentions")]
         public Optional<AllowedMentions> AllowedMentions { get; set; }
+        [JsonProperty("referenced_message")]
+        public Optional<Message> ReferencedMessage { get; set; }
     }
 }