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/Entities/Users/RestWebhookUser.cs

@@ -95,5 +95,7 @@ namespace Discord.Rest
         IVoiceChannel IVoiceState.VoiceChannel => null;
         /// <inheritdoc />
         string IVoiceState.VoiceSessionId => null;
+        /// <inheritdoc />
+        bool IVoiceState.IsStream => false;
     }
 }