Adding Support for kubectl ExecCredential output

**Why:**

* Allows you to use the `heptio-authenticator-aws` binary in your
`~/.kube/config` as an external authProvider

**Usage:**

```yaml

users:
- name: user
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1alpha1
      command: heptio-authenticator-aws
      args:
        - "token"
        - "--cluster-id"
        - "cluster-id"
```

**This change addresses the need by:**

* closes kubernetes-sigs#64

Signed-off-by: Christopher Hein <me@christopherhein.com>

cmd/heptio-authenticator-aws/token.go

@@ -58,7 +58,9 @@ var tokenCmd = &cobra.Command{
 			fmt.Fprintf(os.Stderr, "could not get token: %v\n", err)
 			os.Exit(1)
 		}
-		fmt.Println(tok)
+
+		enc := gen.FormatJSON(tok)
+		fmt.Println(enc)
 	},
 }
 

pkg/token/token.go

@@ -32,6 +32,8 @@ import (
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/sts"
 	"github.com/heptio/authenticator/pkg/arn"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	clientauthv1alpha1 "k8s.io/client-go/pkg/apis/clientauthentication/v1alpha1"
 )
 
 // Identity is returned on successful Verify() results. It contains a parsed
@@ -125,6 +127,8 @@ type Generator interface {
 	Get(string) (string, error)
 	// GetWithRole creates a token by assuming the provided role, using the credentials in the default chain.
 	GetWithRole(clusterID, roleARN string) (string, error)
+	// FormatJSON returns the client auth formatted json for the execcredential auth
+	FormatJSON(string) string
 }
 
 type generator struct {
@@ -180,6 +184,21 @@ func (g generator) GetWithRole(clusterID string, roleARN string) (string, error)
 	return v1Prefix + base64.RawURLEncoding.EncodeToString([]byte(presignedURLString)), nil
 }
 
+// FormatJSON formats the json to support 1.10 external authProvider
+func (g generator) FormatJSON(token string) string {
+	execInput := &clientauthv1alpha1.ExecCredential{
+		TypeMeta: metav1.TypeMeta{
+			APIVersion: "client.authentication.k8s.io/v1alpha1",
+			Kind:       "ExecCredential",
+		},
+		Status: &clientauthv1alpha1.ExecCredentialStatus{
+			Token: token,
+		},
+	}
+	enc, _ := json.Marshal(execInput)
+	return string(enc)
+}
+
 // Verifier validates tokens by calling STS and returning the associated identity.
 type Verifier interface {
 	Verify(token string) (*Identity, error)