Add a type for requiring/awaiting state in the cluster

[ananace]

After discussion in #34, adding a type that will check for requested state could be useful in ensuring that resource application only happens where/when reasonable. For example; kubectl_apply-ing an operator, and then waiting for it to create its managed CRDs before attempting to use them.

The MVP for this would involve a type that checks the content of a JSON-path of an object, comparing it against a given value - perhaps with a user-provided operator. The Kubernetes match expressions (key, operator, values - with its operators of In/NotIn/Exists/DoesNotExist/Gt/Lt) might be a good design to copy, since it will be familiar to how things are done inside the cluster.