Add configurable exception case to prefer-some-in-iteration

[anderseknert]

There are a few cases where some .. in can't directly replace [_] iteration. In the gatekeeper-library repo, they do a whole lot of:

input_containers[c] {
    c := input.review.object.spec.containers[_]
}
input_containers[c] {
    c := input.review.object.spec.initContainers[_]
}
input_containers[c] {
    c := input.review.object.spec.ephemeralContainers[_]
}

Which without unconditional-assignment becomes:

input_containers[input.review.object.spec.containers[_]]

input_containers[input.review.object.spec.initContainers[_]]

input_containers[input.review.object.spec.ephemeralContainers[_]]

Using some ... in doesn't work in that location, so the alternative would be:

input_containers[c] {
    some c in input.review.object.spec.ephemeralContainers
}

# or

input_containers contains c if some c in input.review.object.spec.ephemeralContainers

But that's arguably an "unconditional assignment" — or at least an unconditional if — too. If one prefers the shorter [_] syntax in this case I think that should be allowed as an option.