Add Cache as another type of context

[gdalle]

At the moment, DI only supports passing additional arguments if they are Constant, that is, their value is not modified by the function.
We also need to support passing Caches, which can be mutated with active (differentiated) values.
Here's a breakdown of how this can work, backend by backend:

• Finite differences: straightforward, active values are just standard floats
• ForwardDiff: preallocate a corresponding cache with Dual numbers, similar to PreallocationTools.jl
• Enzyme: mark the caches with (Batch)Duplicated

[gdalle]

Implemented and tested for finite difference backends in #587