Toolchain configuration support

[kzadorozhny]

Description of the problem / feature request:

Right now platform specific kubectl and kustomize are downloaded within repository rule. This method makes support of additional operation systems or execution environments very time

Feature requests: what underlying problem are you trying to solve with this feature?

• Support additional operating systems. NixOS Support for NixOS #54
• Support wide variety of execution environments Allow using custom kubectl binary #46
• Support symlinking system kubeconfig Symlink config #22
• Running Kubernetes tests on GCP Symlink config #22

Proposed design

K8sInfo = provider(
    doc = "Information about how to invoke the Kubernetes tools.",
    fields = {
        "kubectl_path": "Path to the kubectl executable.",
        "kubectl_target": "A kubectl executable target.",
        "kustomize_path": "Path to the kustomize executable.",
        "kustomize_target": "A kustomize executable target.",
        "kubeconfig_path": "Path to kubeconfig file.",
        "kubeconfig_target": "A kubeconfig file target.",
        "kubeconfig_cluster": "Kubernetes cluster name.",
        "kubeconfig_namespace": "Kubernetes namespace.",
        "kubeconfig_user": "Kubernetes user name.",
    },
)

[nickschaap]

This should be fixed in 1.0.0. The kubectl module extension will provide default toolchains but you can define your own and register it inside the your repository MODULE.bazel file or by passing it via --extra_toolchains