[IPickerListItem interface] Add size option to avatar prop and allow to display avatar in selected element

[robertp-lc]

Description

Follow-up request after #1349

1. When using new avatar prop in PickerListItem, there's no option to control the size of the avatar via additional prop. Perhaps avatar could have size included?
2. Default avatar shape is not a circle, it's more like a rectangle with rounded corners. I believe that the default shape should be set to a circle.
3. Would love to see the option to display avatar for selected element, without the need of using customElement prop.

Example screen with the new avatar prop used:

Design

No response