Fixes#8755
Since I was updating the interface, i also added the missing `GetNamespace()`.
Depending upon how you look at it, this is a breaking change since it adds methods to the exported interface `api.ConfigEntry`. Given that you cannot define your own config entry kinds, and all of the machinery of the `api.Client` acts like a factory to construct the canned ones from the rest of the module, this feels like it's not a problematic change as it would only break someone who had reimplemented the `ConfigEntry` interface themselves for no apparent utility?
