Pass down the "allow" property to custom widgets

[SimonMulquin]

Describe the problem to be solved

I am making a widget to allow the user to take calls directly in Grist, to do so, I need to ask the user to allow camera and microphone.

Describe the solution you would like

To do that, I need the widget's iframe to pass down the "allow" property so I can in turn pass it down to the call's iframe.

See an example here: Servant-Cities/grist-integrations@828143a

It would be great to extend grist-plugin-api to let the widget request permissions just like we do when requesting document access.