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
utils.email.send_email('to', 'subject', 'content')
        _, call_kwargs = send_email_test.call_args
        assert call_kwargs['from_email'] == 'from@test.com'
        assert not mock_send_email_smtp.called
    def test_build_mime_message(self):
        mail_from = 'from@example.com'
        mail_to = 'to@example.com'
        subject = 'test subject'
        html_content = '<html>Test</html>'
        custom_headers = {'Reply-To': 'reply_to@example.com'}
        msg, recipients = build_mime_message(
            mail_from=mail_from,
            to=mail_to,
            subject=subject,
            html_content=html_content,
            custom_headers=custom_headers,
        )
        assert 'From' in msg
        assert 'To' in msg
        assert 'Subject' in msg
        assert 'Reply-To' in msg
        assert [mail_to] == recipients
        assert msg['To'] == ','.join(recipients)
class TestEmailSmtp(unittest.TestCase):