Fix/Implement various invite-related behaviors (#1023)

* Initial support for invite member count arg

* Fix IDiscordClient#GetInviteAsync behavior

- Previously, the GetInviteAsync method would return null since it couldn't be parsed as a simple RestInvite object. The object should be a RestInviteMetadata instead.

* Fix methods that didn't comply with the interface

* Change with_counts REST behaviour

* Remove unnecessary JSON prop

* Remove AcceptAsync

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1796,8 +1796,8 @@ namespace Discord.WebSocket
         async Task<IReadOnlyCollection<IConnection>> IDiscordClient.GetConnectionsAsync(RequestOptions options)
             => await GetConnectionsAsync().ConfigureAwait(false);
 
-        async Task<IInvite> IDiscordClient.GetInviteAsync(string inviteId, RequestOptions options)
-            => await GetInviteAsync(inviteId).ConfigureAwait(false);
+        async Task<IInvite> IDiscordClient.GetInviteAsync(string inviteId, bool withCount, RequestOptions options)
+            => await GetInviteAsync(inviteId, withCount, options).ConfigureAwait(false);
 
         Task<IGuild> IDiscordClient.GetGuildAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IGuild>(GetGuild(id));