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
final_mock = mock.Mock()
        side_effects = [SMTPServerDisconnected(), SMTPServerDisconnected(), final_mock]
        mock_smtp.side_effect = side_effects
        msg = MIMEMultipart()
        utils.email.send_mime_email('from', 'to', msg, dryrun=False)
        mock_smtp.assert_any_call(
            host=conf.get('smtp', 'SMTP_HOST'),
            port=conf.getint('smtp', 'SMTP_PORT'),
            timeout=conf.getint('smtp', 'SMTP_TIMEOUT'),
        )
        assert mock_smtp.call_count == side_effects.index(final_mock) + 1
        assert not mock_smtp_ssl.called
        assert final_mock.starttls.called
        final_mock.sendmail.assert_called_once_with('from', 'to', msg.as_string())
        assert final_mock.quit.called