Extract stash package

[mamachanko]

The reconcilers package is large. We would like to extract everything related to stashing into a stash package.

We don't want to break anyone's code. That means we'd like to alias the types in the reconcilers package for backwards-compatibility.

⚡ Some of the types, most prominently Stasher are generic. However, generic types cannot be aliased unless GOEXPERIMENT=aliastypeparams is set or until runtime depends on Go 1.24 (see alias names). That means this is blocked until Go 1.24 is out and we require it.

validation.WithRecursive can start using a Stasher then.

[mamachanko]

Go 1.24 is out with generic type aliases.

[scothis]

This will probably be actionable in 6 months once k8s bumps to require go 1.24.