Add "inset" prop to remove internal paddings

[acd02]

There is a common use case in Polaris where users want to have a picture within the Dialog or Drawer (or Popover) component that takes up the full width.

Currently, achieving this requires overriding the internal padding of the component. To prop would make this use case easier and more intuitive.

When set to true, this prop would remove the internal padding of the component, allowing for a full-width image or content.

### Tasks
- [ ] https://github.com/adevinta/spark/issues/1652
- [ ] https://github.com/adevinta/spark/issues/1653
- [ ] https://github.com/adevinta/spark/issues/1654