[Feature] Styling Image symbols on scatter chart

### What problem does this feature solve?

I am currently trying to print a delegator/supply distribution chart and the image symbols I am using is loaded from keybase. I want them to display as circles instead of squares. It does not look refined. I would like for there to be a way to modify the border radius of the image.

### What does the proposed API look like?

{
type: 'scatter',
data: mappedValidators,
symbol: 'image://'
symbolBorderRadius: 5

}