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/operators/email.py
"""
    Sends an email.
    :param to: list of emails to send the email to. (templated)
    :type to: list or string (comma or semicolon delimited)
    :param subject: subject line for the email. (templated)
    :type subject: str
    :param html_content: content of the email, html markup
        is allowed. (templated)
    :type html_content: str
    :param files: file names to attach in email (templated)
    :type files: list
    :param cc: list of recipients to be added in CC field
    :type cc: list or string (comma or semicolon delimited)
    :param bcc: list of recipients to be added in BCC field
    :type bcc: list or string (comma or semicolon delimited)
    :param mime_subtype: MIME sub content type
    :type mime_subtype: str
    :param mime_charset: character set parameter added to the Content-Type
        header.
    :type mime_charset: str
    """
    template_fields = ('to', 'subject', 'html_content', 'files')
    template_fields_renderers = {"html_content": "html"}
    template_ext = ('.html',)
    ui_color = '#e6faf9'
    def __init__(