Add client auth plugin framework for kubectl with GCP auth plugin.

pkg/client/unversioned/clientcmd/client_config.go

@@ -172,6 +172,9 @@ func getUserIdentificationPartialConfig(configAuthInfo clientcmdapi.AuthInfo, fa
 		mergedConfig.Username = configAuthInfo.Username
 		mergedConfig.Password = configAuthInfo.Password
 	}
+	if configAuthInfo.AuthProvider != nil {
+		mergedConfig.AuthProvider = configAuthInfo.AuthProvider
+	}
 
 	// if there still isn't enough information to authenticate the user, try prompting
 	if !canIdentifyUser(*mergedConfig) && (fallbackReader != nil) {
@@ -212,8 +215,8 @@ func makeServerIdentificationConfig(info clientauth.Info) restclient.Config {
 func canIdentifyUser(config restclient.Config) bool {
 	return len(config.Username) > 0 ||
 		(len(config.CertFile) > 0 || len(config.CertData) > 0) ||
-		len(config.BearerToken) > 0
-
+		len(config.BearerToken) > 0 ||
+		config.AuthProvider != nil
 }
 
 // Namespace implements KubeConfig