Make Call Waiting an optional feature

### Your use case

#### What would you like to do?

I would like the option to turn Call Waiting off.

In essence: if I'm on a call already, and another user calls me, they should get a busy tone. instead of successfully ringing my device and disrupting my ongoing call.

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

Where I'm from, this is unexpected behaviour. Call waiting for regular phone calls is a feature that you need to explicitly turn on. No service provider turns it on for you by default.

VoIP-enabled apps like Whatsapp also went for the busy tone route, instead of Call Waiting.

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

A toggle in the "Voice & Video" settings menu seems like the best solution.

### Have you considered any alternatives?

_No response_

### Additional context

I thought this was a bug in the app and was about to report it as such, until I investigated #5829 properly.