adopt async_dispatch for slack collection

[zzstoatzz]

related to #15008

first of many PRs to define explicit sync / async interfaces where we previously had @sync_compatible

In [1]: from prefect_slack import SlackWebhook

In [2]: s = SlackWebhook(url="https://hooks.slack.com/services/xxx/xxx/xxx")

# all of the below work

In [3]: s.notify("oi")

In [4]: await s.notify_async("oi")

In [5]: await s.notify("oi")