Add CustomDataStore.Exists, alongside helper method on Player

[xCynDev]

This PR adds CustomDataStore.Exists(), which allows you to check if a CustomDataStore instance exists on the given Player, without creating one like CustomDataStore.GetOrAdd() would.

Since there currently is no way to check for that, I figured I'd make a PR.

It also adds CustomDataStore<TStore>.Exists() and Player.HasDataStore<TStore>() as helper methods.

Made a new PR to merge into data-store-performance instead of master as requested by @x3rt

[x3rt]

Thank you!