[Feature] Refactor SendMessageAsync & SendFile(s)Async methods & modify webhook message attachments (#2609)

* initial commit

* resolve merge conficts

* code review suggestions

src/Discord.Net.Rest/API/Rest/CreateMessageParams.cs

@@ -6,7 +6,7 @@ namespace Discord.API.Rest
     internal class CreateMessageParams
     {
         [JsonProperty("content")]
-        public string Content { get; }
+        public Optional<string> Content { get; set; }
 
         [JsonProperty("nonce")]
         public Optional<string> Nonce { get; set; }
@@ -31,10 +31,5 @@ namespace Discord.API.Rest
 
         [JsonProperty("flags")]
         public Optional<MessageFlags> Flags { get; set; }
-
-        public CreateMessageParams(string content)
-        {
-            Content = content;
-        }
     }
 }