clientv3: dial with context when creating authenticator

Otherwise, "grpc.Dial" blocks when "grpc.WithTimeout" dial
option gets deprecated.

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

clientv3/auth.go

@@ -216,8 +216,8 @@ func (auth *authenticator) close() {
 	auth.conn.Close()
 }
 
-func newAuthenticator(endpoint string, opts []grpc.DialOption, c *Client) (*authenticator, error) {
-	conn, err := grpc.Dial(endpoint, opts...)
+func newAuthenticator(ctx context.Context, endpoint string, opts []grpc.DialOption, c *Client) (*authenticator, error) {
+	conn, err := grpc.DialContext(ctx, endpoint, opts...)
 	if err != nil {
 		return nil, err
 	}