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
mock_smtp.side_effect = SMTPServerDisconnected()
        msg = MIMEMultipart()
        custom_retry_limit = 10
        custom_timeout = 60
        with conf_vars(
            {
                ('smtp', 'smtp_retry_limit'): str(custom_retry_limit),
                ('smtp', 'smtp_timeout'): str(custom_timeout),
            }
        ):
            with pytest.raises(SMTPServerDisconnected):
                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=custom_timeout
        )
        assert not mock_smtp_ssl.called
        assert mock_smtp.call_count == 10
    @mock.patch('smtplib.SMTP_SSL')
    @mock.patch('smtplib.SMTP')
    def test_send_mime_partial_failure(self, mock_smtp, mock_smtp_ssl):