Mark guild optional in invite & general invite improvement (#1094)

* Mark guild as optional for invite

* Mark partial InviteMetadata members as Optional<T>

* Some of them aren't sent when requesting through the general GET invite endpoint

* Remove GetInviteParams

* It was kinda stupid in the first place, might as well always get the count instead of having to ask the user whether they want the two fields filled or not.

* Add ChannelType property

* Add vanity invite support

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

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 using System;
 
@@ -9,15 +9,15 @@ namespace Discord.API
         [JsonProperty("inviter")]
         public User Inviter { get; set; }
         [JsonProperty("uses")]
-        public int Uses { get; set; }
+        public Optional<int> Uses { get; set; }
         [JsonProperty("max_uses")]
-        public int MaxUses { get; set; }
+        public Optional<int> MaxUses { get; set; }
         [JsonProperty("max_age")]
-        public int MaxAge { get; set; }
+        public Optional<int> MaxAge { get; set; }
         [JsonProperty("temporary")]
         public bool Temporary { get; set; }
         [JsonProperty("created_at")]
-        public DateTimeOffset CreatedAt { get; set; }
+        public Optional<DateTimeOffset> CreatedAt { get; set; }
         [JsonProperty("revoked")]
         public bool Revoked { get; set; }
     }