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
#
#
#
import os
import tempfile
import unittest
from email.mime.application import MIMEApplication
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from smtplib import SMTPServerDisconnected
from unittest import mock
import pytest
from airflow import utils
from airflow.configuration import conf
from airflow.utils.email import build_mime_message, get_email_address_list
from tests.test_utils.config import conf_vars
EMAILS = ['test1@example.com', 'test2@example.com']
send_email_test = mock.MagicMock()
class TestEmail(unittest.TestCase):