[Feature] Polls (#2902)

no breakings hopefully

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

@@ -48,6 +48,9 @@ namespace Discord.API.Rest
         [JsonProperty("applied_tags")]
         public Optional<ulong[]> AppliedTags { get; set; }
 
+        [JsonProperty("poll")]
+        public Optional<CreatePollParams> Poll { get; set; }
+
         public IReadOnlyDictionary<string, object> ToDictionary()
         {
             var d = new Dictionary<string, object>();
@@ -80,6 +83,11 @@ namespace Discord.API.Rest
                 payload["thread_name"] = ThreadName.Value;
             if (AppliedTags.IsSpecified)
                 payload["applied_tags"] = AppliedTags.Value;
+            if (Flags.IsSpecified)
+                payload["flags"] = Flags.Value;
+            if (Poll.IsSpecified)
+                payload["poll"] = Poll.Value;
+
 
             var json = new StringBuilder();
             using (var text = new StringWriter(json))