Add TLS support to exec authenticator plugin

https://github.com/kubernetes/community/blob/master/contributors/design-proposals/auth/kubectl-exec-plugins.md#tls-client-certificate-support

Allows exec plugin to return raw TLS key/cert data. This data populates
transport.Config.TLS fields.
transport.Config.TLS propagates custom credentials using
tls.Config.GetClientCertificate callback.
On key/cert rotation, all connections using old credentials are
closed

Kubernetes-commit: cd89f9473faa60c15b8e9d223e5c4f9dab53627a

pkg/apis/clientauthentication/types.go

@@ -57,7 +57,14 @@ type ExecCredentialStatus struct {
 	// +optional
 	ExpirationTimestamp *metav1.Time
 	// Token is a bearer token used by the client for request authentication.
+	// +optional
 	Token string
+	// PEM-encoded client TLS certificate.
+	// +optional
+	ClientCertificateData string
+	// PEM-encoded client TLS private key.
+	// +optional
+	ClientKeyData string
 }
 
 // Response defines metadata about a failed request, including HTTP status code and

pkg/apis/clientauthentication/v1alpha1/types.go

@@ -52,12 +52,20 @@ type ExecCredentialSpec struct {
 }
 
 // ExecCredentialStatus holds credentials for the transport to use.
+//
+// Token and ClientKeyData are sensitive fields. This data should only be
+// transmitted in-memory between client and exec plugin process. Exec plugin
+// itself should at least be protected via file permissions.
 type ExecCredentialStatus struct {
 	// ExpirationTimestamp indicates a time when the provided credentials expire.
 	// +optional
 	ExpirationTimestamp *metav1.Time `json:"expirationTimestamp,omitempty"`
 	// Token is a bearer token used by the client for request authentication.
 	Token string `json:"token,omitempty"`
+	// PEM-encoded client TLS certificates (including intermediates, if any).
+	ClientCertificateData string `json:"clientCertificateData,omitempty"`
+	// PEM-encoded private key for the above certificate.
+	ClientKeyData string `json:"clientKeyData,omitempty"`
 }
 
 // Response defines metadata about a failed request, including HTTP status code and

pkg/apis/clientauthentication/v1alpha1/zz_generated.conversion.go

@@ -99,6 +99,8 @@ func Convert_clientauthentication_ExecCredentialSpec_To_v1alpha1_ExecCredentialS
 func autoConvert_v1alpha1_ExecCredentialStatus_To_clientauthentication_ExecCredentialStatus(in *ExecCredentialStatus, out *clientauthentication.ExecCredentialStatus, s conversion.Scope) error {
 	out.ExpirationTimestamp = (*v1.Time)(unsafe.Pointer(in.ExpirationTimestamp))
 	out.Token = in.Token
+	out.ClientCertificateData = in.ClientCertificateData
+	out.ClientKeyData = in.ClientKeyData
 	return nil
 }
 
@@ -110,6 +112,8 @@ func Convert_v1alpha1_ExecCredentialStatus_To_clientauthentication_ExecCredentia
 func autoConvert_clientauthentication_ExecCredentialStatus_To_v1alpha1_ExecCredentialStatus(in *clientauthentication.ExecCredentialStatus, out *ExecCredentialStatus, s conversion.Scope) error {
 	out.ExpirationTimestamp = (*v1.Time)(unsafe.Pointer(in.ExpirationTimestamp))
 	out.Token = in.Token
+	out.ClientCertificateData = in.ClientCertificateData
+	out.ClientKeyData = in.ClientKeyData
 	return nil
 }