Allow callers to create VariableSpecs

[ti-mo]

For testing purposes, it would be really convenient if the caller could instantiate their own VariableSpecs. Cilium currently uses something like this:

var _ VariableSpec = (*ebpf.VariableSpec)(nil)

type VariableSpec interface {
	MapName() string
	Offset() uint64
	Size() uint64
	Get(out any) error
	Constant() bool
}

This requires converting ebpf.VariableSpecs to our Cilium-internal VariableSpec since that's what our internal functions take to allow tests to provide their own.

This issue is for adding a NewVariableSpec or similar with some sanity checks to allow users to create their own. As long as the accesses performed by the VariableSpec are valid, I guess this isn't something we should really prevent. Pay attention to CollectionSpec/VariableSpec.Copy() behaviour.

[lmb]

Not sure where we had the discussion, but I think the answer is to export the fields somehow. That would make it in line with our other specs as well.