Optional rustls support

[jnicholls]

Some Rust users in the community are not interested in having OpenSSL as a dependency. Especially just for the support of loading certificate/key data from kubeconfig files. reqwest already supports rustls as an optional feature instead of using native-tls. You would simply need to switch the cert/key loading to only use reqwest Certificate/Identity APIs, and potentially rustls::internal::pemfile if necessary, rather than the openssl crate.

[clux]

Yeah, I would be very happy to have rustls as a feature (even having it as the default feature if it works well). Reqwest dependency is easy to switch out. My time is somewhat limited here, but would gladly take a PR for rustls support in the config module.

[clux]

@jnicholls I've tried following your plan in #114 using some of @ctron 's linked commit (ty). Unfortunately, have not been able to make it communicate with a cluster yet (just getting 403s back). My pem input + cert bundles all seem sensible afaikt, but have very little knowledge in this area. Any help to make this work would be appreciated.

[jnicholls]

@clux Thanks for jumping into this! You are amazing for creating and supporting this great library and just wanted to start with a big thank you.

This afternoon I will find some time to jump in here and see if I can lend a hand!

[clux]

Appreciate that! Just a heads up that the rustls issue is actually more or less resolved! Just need to tidy it up a bit for a release and port the oauth module.

Any more of the help that you've already jump on or want to jump on is still very much appreciated though!

[clux]

Initial support in master. Will release a version with it soon. But it's probably not usable for gke yet: see #120

[clux]

Released in 0.25.0 along with a great pr to help #120