Add a setting to select a custom notification sound

### Your use case

#### What would you like to do?

I am _personally_ not a fan of the default Notification sound for Element. To me it's harsh.

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

I would like to globally customize a sound for notifications in Element.

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

Go to iOS Settings > Notifications > Element and change the default tone.

### Have you considered any alternatives?

To my knowledge, there is no way to customize notification sounds for Element iOS at this time.

### Additional context

_No response_