Implement Acknowledgement Timeout for Alerts

[mastercactapus]

What problem would you like to solve? Please describe: There's an issue where alerts could potentially get acknowledged and then forgotten indefinitely.

Describe the solution you'd like: Add an option for admins to set an acknowledgement timeout limit (in hours or days). Alerts that are acknowledged but do not get closed within this limit should return to an unacknowledged state, prompting a fresh round of notifications.

Describe alternatives you've considered: The current alternative is to manually close out old alerts, but this is not efficient, especially on a larger scale.

Additional context: This would also help alleviate system processing loads from old forgotten alerts, and reduce confusion for users seeing their new alerts get suppressed due to deduplication from similar open alerts.

[mastercactapus]

For this, let's plan to implement it at the Service level as an opt-in feature.

That way teams can leverage it if they need to for their specific use case. For the admin-level handling of stale acknowledged alerts, we'll handle that here: https://github.com/target/goalert/issues/3397