Send without sound

### Your use case

#### What would you like to do?

Send a message and only trigger a silent notification at the receiving end.

#### Why would you like to do it?

To be less obnoxious.

#### How would you like to achieve it?

It should ideally be cross-platform, but I think we can figure out the gist of it on Android first and port it later. Should be indicated by a metadata field in the message metadata, which is read by the client before sending a notification event.

### Have you considered any alternatives?

_No response_

### Additional context

Telegram has this really amazing feature to let me send messages without triggering the notification ping at the receiving device (just a silent notification).

It is very useful in many scenarios, not least to be nice about late messages to parents and alike who have not understood how to work the 'do not disturb' feature in their phones.

This works cross-platform, so I assume it needs to be supported via a well-known metadata field in the message.