say I want to have a phone peripheral that's a peripheral device and an external battery integrated into a phone case. in 2023 where everything is USB-C do I actually have to implement USB-PD or can I implement the older (and much simpler) accessory charger adapter spec from USB-OTG? the usb people say that's not allowed with USB-C but surely it must be if USB-C to USB-A adapters exist?