Expose all auth-provider config keys

`kubectl config unset users.foo.auth-provider.azure.refresh-token` dies with "error: unable to parse one or more field values" because `pkg/cmd/config/navigation_step_parser.go` cannot deal with pointers and `*AuthProviderConfig` is one.

**What would you like to be added**:

Allow `kubectl config` to un/set any field in the YAML, particularly `auth-provider` settings.

**Why is this needed**:

When giving people access to a cluster it is often necessary to deal with authentication issues, and sending them command lines is easier for everybody than instructions on how to edit the kubeconfig by hand.