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
e_from: str, e_to: List[str], mime_msg: MIMEMultipart, conn_id: str = "smtp_default", dryrun: bool = False
) -> None:
    """Send MIME email."""
    smtp_host = conf.get('smtp', 'SMTP_HOST')
    smtp_port = conf.getint('smtp', 'SMTP_PORT')
    smtp_starttls = conf.getboolean('smtp', 'SMTP_STARTTLS')
    smtp_ssl = conf.getboolean('smtp', 'SMTP_SSL')
    smtp_retry_limit = conf.getint('smtp', 'SMTP_RETRY_LIMIT')
    smtp_timeout = conf.getint('smtp', 'SMTP_TIMEOUT')
    smtp_user = None
    smtp_password = None
    smtp_user, smtp_password = None, None
    if conn_id is not None:
        try:
            from airflow.hooks.base import BaseHook
            conn = BaseHook.get_connection(conn_id)
            smtp_user = conn.login
            smtp_password = conn.password
        except AirflowException:
            pass