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
#
#
#
import collections.abc
import logging
import os
import smtplib
import warnings
from email.mime.application import MIMEApplication
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.utils import formatdate
from typing import Any, Dict, Iterable, List, Optional, Tuple, Union
from airflow.configuration import conf
from airflow.exceptions import AirflowConfigException, AirflowException
log = logging.getLogger(__name__)
def send_email(
    to: Union[List[str], Iterable[str]],
    subject: str,
    html_content: str,