Add Async support for SMTP Notifier

[ferruzzi]

Following up on #53831 and part of #55237

Dags used for manual testing are below. email_operator is a "control" to make sure the SMPT connection is working as expected, email_as_success_callback confirms that existing synchronous notifier is still working, and email_as_success_callback tests the new async functionality

with DAG("email_as_success_callback"):
    BashOperator(
        task_id='example_task',
        bash_command="echo 'Hello World'",
        on_success_callback=[
            SmtpNotifier(to="dferruzzi@gmail.com", subject="Hello from Airflow on_success", html_content="IT WORKS!")
        ],
    )


with DAG('email_operator'):
    EmailOperator(
        task_id='send_email_to_sender',
        to='dferruzzi@gmail.com',
        subject="hello from airflow dag",
        html_content="hello from airflow dag"
    )

with DAG(
    "email_notifier_as_deadline",
    deadline=DeadlineAlert(
        reference=DeadlineReference.DAGRUN_QUEUED_AT,
        interval=timedelta(seconds=1),
        callback=AsyncCallback(
            callback_callable=SmtpNotifier,
            kwargs={
                "to": "dferruzzi@gmail.com",
                "subject": "Airflow deadline missed",
                "html_content": "Your dag ran too long!",
            }
        )
    )
):
    BashOperator(task_id='sleep_for_ten_seconds', bash_command='sleep 10')

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[ramitkataria]

A couple of templating-related suggestions and some nits but otherwise looks good to me!