Allow context, cluster, and user names to be specified

[svend]

Allow the desired context name, cluster name, and user name to be specified when loading the Kubernetes configuration.

Do you prefer an option struct or separate arguments?

option struct

#[derive(Default)]
pub struct ConfigOptions {
    pub context: Option<String>,
    pub cluster: Option<String>,
    pub user: Option<String>,
}

pub fn load_kube_config_with(options: ConfigOptions)

usage:

let config_opts = config::ConfigOptions {
    context: "mycontext",
    ..Default::default()
};

let kubeconfig = config::load_kube_config_with(config_opts)?;

arguments

pub fn load_kube_config_with(
    context: Option<String>,
    cluster: Option<String>,
    user: Option<String>,
)

usage:

let kubeconfig = config::load_kube_config_with(Some("mycontext"), None, None)?;

[ynqa]

@svend LGTM. Sorry for response delay and thanks for contributions (fixing conflicts also) again!