create a config feature in kube

[clux]

We currently have a "private" __config feature in kube to track the config related dependencies.

I would like to expose this properly so that people can use kube with only the config feature to deal with kubeconfig related parsing.

This would require:

• having checks for #[cfg(feature = "config") for the config module
• having config feature depend on internal __non_core feature
• having checks for #[cfg(feature = "config") or #[cfg(feature = "client") (or possibly feature = __non_core?) for overlapping deps
• having the error enum be more feature flagged (currently entire module is gated on client feature)
• a small test example that parser kubeconfig without features (needs some feature-re-export hackery in examples/Cargo.toml, plus a circleci test line)

You should not be able to get the client feature without the config feature (already ensured by having __config feature listed as a dependency of the client feature), but the other way around should work.

[clux]

released in 0.56