provide rustls implementation to sign request to gke

[davidB]

related #120

I refactor a bit the code:

• to reduce / to remove the duplication between fragment for native-tls and rustls-tls
• to test the signing function

for my quick test native-tls and rustls-tls behave the same, but

• I fail to use rustls as is with my test gke server because the server url was with an ip (like https://35.187.173.xxx/) and that is not supported by rustls see Missing support for ipv4/ipv6 subjects · Issue #56 · ctz/hyper-rustls (I workaround by modifing etc/hosts and .kube/config to use the url https://kubernetes.default.
• require GOOGLE_APPLICATION_CREDENTIALS is not friendly and the behavior of kubectl (nor go-client), I will investigate to make a PR later to fix that
• I was not able to connect my cluster after expiration of my token (same error by both tls)

So I guess that the lib is not usable as is with gke

FYI, currently in my kubectl plugin, I don't let kube-rs manage token (and refresh) because

• it doesn't update .kube/config after refresh (and that cause trouble with kubectl (and other
  plugin))
• it doesn't handle well some case
  So I call kubectl cluster-infobefore.

async fn load_kube_config() -> Result<config::Configuration> {
    //HACK force refresh token by calling "kubectl cluster-info before loading configuration"
    use std::process::Command;
    let output = Command::new("kubectl")
        .arg("cluster-info")
        .output()
        .with_context(|| "failed to executed 'kubectl cluster-info'")?;
    if !output.status.success() {
        return Err(anyhow!("`kubectl cluster-info` failed with: {:?}", &output));
    }
    config::load_kube_config()
        .await
        .with_context(|| "failed to load kubeconfig")
}

Sorry for long text

[clux]

I refactor a bit the code to reduce / to remove the duplication

It's very good, and looks clean. Thank you so much. Have tested it against EKS and will merge right away.

• I fail to use rustls as is with my test gke server because the server url was with an ip (like https://35.187.173.xxx/) and that is not supported by rustls see Missing support for ipv4/ipv6 subjects · Issue #56 · ctz/hyper-rustls (I workaround by modifing etc/hosts and .kube/config to use the url https://kubernetes.default.

Oh wow. That's annoying. Thanks for finding the root issue.

• require GOOGLE_APPLICATION_CREDENTIALS is not friendly and the behavior of kubectl (nor go-client), I will investigate to make a PR later to fix that

👍

FYI, currently in my kubectl plugin, I don't let kube-rs manage token (and refresh) because it doesn't update .kube/config after refresh (and that cause trouble with kubectl (and other plugin)) [..] so I call kubectl cluster-infobefore.

Do you feel that perhaps we should patch the ~/.kube/config file with refresh tokens?

[clux]

Opened part of your workaround as a bug on here: #153