Implement idempotent patching in a helper package

[dlipovetsky]

Idempotence: An External Patch Extension must only return patches if changes to the templates are required, i.e. unnecessary patches when the template is already in the desired state must be avoided.
-- https://cluster-api.sigs.k8s.io/tasks/experimental-features/runtime-sdk/implement-topology-mutation-hook#patch-extension-guidelines

We want patches to be idempotent.

Right now, this burden is the individual patch handlers, namely, the Mutate method defined in the MetaMutator interface).

I think we should implement idempotent patching in a package that all handlers can use, for at least these reasons:

1. Idempotence is hard to get right
2. Idempotent patching can be implemented in a way that's not specific to any one handler