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/Entities/Guilds/GuildHelper.cs

@@ -210,6 +210,12 @@ namespace Discord.Rest
             var models = await client.ApiClient.GetGuildInvitesAsync(guild.Id, options).ConfigureAwait(false);
             return models.Select(x => RestInviteMetadata.Create(client, guild, null, x)).ToImmutableArray();
         }
+        public static async Task<RestInviteMetadata> GetVanityInviteAsync(IGuild guild, BaseDiscordClient client,
+            RequestOptions options)
+        {
+            var model = await client.ApiClient.GetVanityInviteAsync(guild.Id, options).ConfigureAwait(false);
+            return RestInviteMetadata.Create(client, guild, null, model);
+        }
 
         //Roles
         public static async Task<RestRole> CreateRoleAsync(IGuild guild, BaseDiscordClient client,