Feature request: with_key taking full closures

[kC0pter]

Adding keys with the with_key method currently only takes function pointers or non-capturing closures that are coercible to function pointers.
It would be a nice addition to change that to full closures to be able to capture locals to add to the printing.

[djc]

The problem here is that ProgressStyle is Clone (which seems like a useful property), and it doesn't look like the keys could be Clone while also using dynamic dispatch.

[1Dragoon]

I think the main thing we're looking for in with_keys closures is that we want to be able to store stateful information that the with_key method can then have read-only access to. So what if we could provide an Option<Box<dyn SomeTrait>> that ProgressState holds on to for us, maybe even ticks it for us, and our with_keys function pointer can then read from it?