Feature request: provide a way to acknowledge a firing alert

When a whole team tries to take an action on multiple alerts during an incident it requires a lot of communication effort to coordinate who is dealing with which alert.
Some systems (like PD) provide a way to acknowlage an alert and assign it to a specific person (usually the on-call person), but that requires to route every alert via such system. Also during an incident often people try to help and volunteer to handle some of alerts, so the usuall routing of alerts might not cover that.
It would be very useful to have some ability to mark an alert as "I'm working on that". This was discussed on the mailing list and one of the proposed solutions was to support auto-expiring silences (once alert is resolved silence is automatically expired regardless of endsAt value), which was previously suggested on #1057 but not accepted.