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. 




airflow/utils/email.py
to: Union[str, Iterable[str]],
    subject: str,
    html_content: str,
    files: Optional[List[str]] = None,
    dryrun: bool = False,
    cc: Optional[Union[str, Iterable[str]]] = None,
    bcc: Optional[Union[str, Iterable[str]]] = None,
    mime_subtype: str = 'mixed',
    mime_charset: str = 'utf-8',
    conn_id: str = "smtp_default",
    from_email: str = None,
    **kwargs,
):
    """
    Send an email with html content
    >>> send_email('test@example.com', 'foo', '<b>Foo</b> bar', ['/dev/null'], dryrun=True)
    """
    smtp_mail_from = conf.get('smtp', 'SMTP_MAIL_FROM')
    mail_from = smtp_mail_from or from_email
    msg, recipients = build_mime_message(
        mail_from=mail_from,
        to=to,
        subject=subject,
        html_content=html_content,
        files=files,
        cc=cc,
        bcc=bcc,
        mime_subtype=mime_subtype,