Short feature/function description
It's only possible to increase or decrease volume right now, as that is what I have managed to implement for existing protocols. For RAOP however, it's possible to set a specific volume and since we decide the volume, we even know what it is. So it would make sense to support both changing volume to a specific level as well as providing in externally.

I will add a new Audio interface for this, likely moving volume_up and volume_down to it over time. It makes more sense to have a dedicated interface (doesn't fit anywhere else) but also allows me to expose more information over time, like output parameters (sample rate, channels, etc.) when I have that information.

What needs to be done?

• Create Audio interface and add set_volume and volume (property) to it
• Implement these in the facade

Is this a breaking change?
No

Anything else worth knowing?
This will only be addition of the interface, no protocol will implement it at this stage.