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
self,
        *,
        to: Union[List[str], str],
        subject: str,
        html_content: str,
        files: Optional[List] = None,
        cc: Optional[Union[List[str], str]] = None,
        bcc: Optional[Union[List[str], str]] = None,
        mime_subtype: str = 'mixed',
        mime_charset: str = 'utf-8',
        conn_id: Optional[str] = None,
        **kwargs,
    ) -> None:
        super().__init__(**kwargs)
        self.to = to
        self.subject = subject
        self.html_content = html_content
        self.files = files or []
        self.cc = cc
        self.bcc = bcc
        self.mime_subtype = mime_subtype
        self.mime_charset = mime_charset
        self.conn_id = conn_id
    def execute(self, context):