Issue:
`custom_headers` is available as an optional keyword argument in `build_mime_message` but it is not exposed in `send_email` or `send_email_smtp`. This prevents using custom headers with the email operator when using the default smtp backend. 

https://github.com/apache/airflow/blob/8e2a0bc2e39aeaf15b409bbfa8ac0c85aa873815/airflow/utils/email.py#L105-L116

Expected:
`send_email` should take optional key word argument `custom_headers` and pass it to `send_email_smtp` if that is the selected backend. 




tests/utils/test_email.py
msg = MIMEMultipart()
        mock_conn = mock.Mock()
        mock_conn.login = 'user'
        mock_conn.password = 'password'
        mock_hook.get_connection.return_value = mock_conn
        utils.email.send_mime_email('from', 'to', msg, dryrun=False, conn_id='smtp_default')
        mock_hook.get_connection.assert_called_with('smtp_default')
        mock_smtp.return_value.login.assert_called_once_with('user', 'password')
        mock_smtp.return_value.sendmail.assert_called_once_with('from', 'to', msg.as_string())
        assert mock_smtp.return_value.quit.called
    @mock.patch('smtplib.SMTP_SSL')
    @mock.patch('smtplib.SMTP')
    def test_send_mime_ssl(self, mock_smtp, mock_smtp_ssl):