Currently we have certain index settings that use the Property.PrivateIndex property, and are thus unsettable from the regular REST API.

We should have a way to determine whether a setting is private from the API. For instance, when re-indexing an index, a user may want to copy the index's settings, but can't copy any private settings, or else the index creation will fail with "Validation Failed: 1: private index setting [index.frozen] can not be set explicitly"