feature: add StartedAt, EndsAt, Elapsed and Remaining to SpotifyGame. (#1414)

* Fixed GetUsersAsync to use MaxUsersPerBatch const as limit instead of MaxMessagesPerBatch.

Requests are now returning up to 1000 guild user entities instead of the previous 100.

* Added StartedAt and EndsAt timespans for SpotifyGame.
They make it possible to expose track's Elapsed and Remaining properties.

* Moved Duration to be initialized within model construction.

* Updated StartedAt and EndsAt comments.

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -38,6 +38,8 @@ namespace Discord.WebSocket
                     AlbumTitle = albumText,
                     TrackTitle = model.Details.GetValueOrDefault(),
                     Artists = model.State.GetValueOrDefault()?.Split(';').Select(x=>x?.Trim()).ToImmutableArray(),
+                    StartedAt = timestamps?.Start,
+                    EndsAt = timestamps?.End,
                     Duration = timestamps?.End - timestamps?.Start,
                     AlbumArtUrl = albumArtId != null ? CDN.GetSpotifyAlbumArtUrl(albumArtId) : null,
                     Type = ActivityType.Listening,