feature: Add SelfStream voice state property (#1369)

* Add self_stream voice state property

Adds the self_stream property which is set true when a user is streaming a video to a voice channel

* use flags for selfstream state instead of its own prop

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

@@ -26,5 +26,7 @@ namespace Discord.API
         public bool SelfMute { get; set; }
         [JsonProperty("suppress")]
         public bool Suppress { get; set; }
+        [JsonProperty("self_stream")]
+        public bool SelfStream { get; set; }
     }
 }