Fix: Integration model from GuildIntegration and added INTEGRATION gateway events (#2168)

* fix integration models; add integration events

* fix description on IGUILD for integration

* fix typo in integration documentation

* fix documentation in connection visibility

* removed public identitiers from app and connection

* Removed REST endpoints that are not part of the API.

* Added documentation for rest integrations

* added optional types

* Fixed rest interaction field with not being IsSpecified

src/Discord.Net.WebSocket/API/Gateway/IntegrationDeletedEvent.cs

@@ -0,0 +1,14 @@
+using Newtonsoft.Json;
+
+namespace Discord.API.Gateway
+{
+    internal class IntegrationDeletedEvent
+    {
+        [JsonProperty("id")]
+        public ulong Id { get; set; }
+        [JsonProperty("guild_id")]
+        public ulong GuildId { get; set; }
+        [JsonProperty("application_id")]
+        public Optional<ulong> ApplicationID { get; set; }
+    }
+}