Add ping feature like in Wire messenger

Purpose of a ping feature is to explicitly signal for a sender to a receiver that he needs your attention. If your messenger graph is trustful that perfectly works, reduce destruction and help to focus on important RSVP things. Actually after using this feature I cannot imaging awesome messenger without it.

The issue names no files, tests, or entry points. Start by locating the messenger or chat entry points and defining how a ping is sent, received, and surfaced; completion criteria are not specified in the issue.

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