Async cache functions (filter/keepOverLimit)

[tekoh]

Which application or package is this feature request for?

discord.js

Feature

I think it would be a good addition/change and I don't see any problems that could come about from it but I could be massively wrong.

Would help to decide whether users should be removed from cache based on if they're an active user from a db query or fetching something from redis etc.

Ideal solution or implementation

Allow cache functions (filter/keepOverlimit) to return Promise

Alternative solutions or implementations

No response

Other context

No response