Provide a "device configuration protocol" for widgets

It's valuable to have consistent UI elements, so I think it could be handy to have a defined protocol that widgets/services can use to share a device configuration API, which Cockpit then creates the UI for from the relevant field types and descriptions, and presents that when the user configures the corresponding widget.

I thought of this [here](https://github.com/bluerobotics/BlueOS/issues/2187), in the context of supporting Ping family sonar devices, and realised it could be extra useful if we did it as a general thing. That was the widget designer can focus on primarily the UI for the main display, and an API for the relevant functionalities, and then Cockpit can handle the UI for configuration stuff.

Start by reviewing this proposal with the linked BlueOS issue 2187 and the Cockpit widget configuration flow. Define the scope of the shared device configuration protocol, including how widgets or services describe fields and how Cockpit presents them. Done should include an agreed general approach that supports the Ping family sonar use case.

Written by the indexing model from the issue text.