[Feature] Set default legend icon for different series types

### What problem does this feature solve?

I have not been able to figure out a way to do this. I think you should be able to declare a default icon type in the legend for each different series, for example:
bar series: star icon
line series: triangle icon
I know that setting a different symbol on a line series will change the icon in the legend, but there is no way to do this for bar charts

### What does the proposed API look like?

bar: {
icon: 'star',
},
line: {
icon: 'triangle',
}