feat: async slack notifier

[dondaum]

Add an asynchronous version of SlackNotifier.

related: #55237

I've added a backwards compatible way to get an Airflow connection using an asynchronous call. In my opinion it was not compatible with before all airflows <3.0.0, which was also confirmed by the CI tests.

Also, I removed @functools.cached_property from an asynchronous method in SlackWebhookHook. In my opinion you can't use @functools.cached_property directly with asynchronous methods because asynchronous methods return Coroutine objects instead of actual values.

I sucessfully tested the SlackNotifier with the following Dag:

from datetime import timedelta
from airflow import DAG
from airflow.sdk.definitions.deadline import AsyncCallback, DeadlineAlert, DeadlineReference
from airflow.providers.slack.notifications.slack import SlackNotifier
from airflow.providers.standard.operators.empty import EmptyOperator
from airflow.sdk import task

with DAG(
    dag_id="deadline_slack_alert_example",
    deadline=DeadlineAlert(
        reference=DeadlineReference.DAGRUN_QUEUED_AT,
        interval=timedelta(seconds=20),
        callback=AsyncCallback(
            SlackNotifier,
            kwargs={
                "text": "🚨 Dag {{ dag_run.dag_id }} missed deadline at {{ deadline.deadline_time }}. DagRun: {{ dag_run }}",
                "channel": "#allgemein",
                "username": "Test App",
            },
        ),
    ),
):
    c = EmptyOperator(task_id="example_task")

    @task()
    def wait():
        import time
        time.sleep(60*5)

    
    c >> wait()

---

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[ferruzzi]

Also, I removed @functools.cached_property from an asynchronous method in SlackWebhookHook. In my opinion you can't use @functools.cached_property directly with asynchronous methods because asynchronous methods return Coroutine objects instead of actual values

Interesting. Good catch.

[shahar1]

LGTM!