Modify `Config` without creating new `CachingClient`

At the moment, the `CachingClient`, which is created using a `Config` can not have its config mutated once it has been created. Thus if say, an override were to be added for a package or namespace, a new client needs to be created, rather than just modifying the config for the existing one. Instead, methods could be added to `CachingClient` that enable updating the config.