AccessTokenAuth should not be private/final

[thadeusb]

We generate access tokens to use with Kubernetes through a custom Vault (https://www.vaultproject.io/) interface. This interface just generates an AccessToken in a string that we then need to pass into Skuber to use, however the interface we implemented is custom and unique to our company, and so adding a pull request to Skuber to just implement a VaultAuth would not work.

Unfortunately, because TokenAuth (and AuthProviderAuth) is marked as a final case class, we cannot override these with our own custom VaultAuth provider. To get around this issue we end up having to call k8sInit for every interaction we need to do, so we can make sure to use fresh credentials.

  val credProvider = CustomVaultCredentialProvider(...)
  def k8s()(implicit a: ActorSystem, m: Materializer): KubernetesClient = {
    val k8sCluster = Cluster(
      server = clusterEndpoint,
      certificateAuthority = Some(Right(clusterCA))
    )
    val k8sContext = Context(
      k8sCluster,
      // This is where the token gets refreshed for skuber.
      TokenAuth(credProvider.refreshAccessToken().getTokenValue))
    )
    val k8sConfig = Configuration(
      clusters = Map("default" -> k8sCluster),
      contexts = Map("default" -> k8sContext),
      currentContext = k8sContext
    )
    k8sInit(k8sConfig)
  }

 // Do something with it
 k8s.get[JobList]...

I am requesting to make AccessTokenAuth subclassable from outside of skuber. That way we can implement a VaultTokenAuth extends AccessTokenAuth and have it work similarly to the existing GcpAuth.