Lazy fetching in countryballs iterator

[laggron42]

The /balls list command currently fetches the entire list of countryballs of a user, with all columns. This ends up taking a lot of time for users with insane amounts of countryballs.

This could be done my modifying CountryballsSource, currently inheriting from the source menus.ListPageSource, to inherit from menus.AsyncIteratorPageSource instead, specifically for lazy fetching of resources.

Reminder that this iterator relies on discord-ext-menus (vendored for interactions/typing), examples on how to implement these subclasses can be found here. Most of the code shouldn't need to be changed.

[laggron42]

I'm lowering the priority on that one, after running some timing checks it turns out the cost is not that high.

It is still an improvement that would be good to have, just not urgent.

[laggron42]

Bumping the priority now, we reached a point where it's needed.