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/RestGuild.cs

@@ -238,6 +238,15 @@ namespace Discord.Rest
         //Invites
         public Task<IReadOnlyCollection<RestInviteMetadata>> GetInvitesAsync(RequestOptions options = null)
             => GuildHelper.GetInvitesAsync(this, Discord, options);
+        /// <summary>
+        ///     Gets the vanity invite URL of this guild.
+        /// </summary>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>
+        ///     A partial metadata of the vanity invite found within this guild.
+        /// </returns>
+        public Task<RestInviteMetadata> GetVanityInviteAsync(RequestOptions options = null)
+            => GuildHelper.GetVanityInviteAsync(this, Discord, options);
 
         //Roles
         public RestRole GetRole(ulong id)
@@ -397,6 +406,9 @@ namespace Discord.Rest
 
         async Task<IReadOnlyCollection<IInviteMetadata>> IGuild.GetInvitesAsync(RequestOptions options)
             => await GetInvitesAsync(options).ConfigureAwait(false);
+        /// <inheritdoc />
+        async Task<IInviteMetadata> IGuild.GetVanityInviteAsync(RequestOptions options)
+            => await GetVanityInviteAsync(options).ConfigureAwait(false);
 
         IRole IGuild.GetRole(ulong id)
             => GetRole(id);