[Feature] Z index styling property of breadcrumbs

What problem does this feature solve?

In treemap chart on zoom the chart is overlapping with breadcrumbs. In case of appending new element to the echart, adding z-index style property to breadcrumbs will allow to differentiate zIndex of separately chart and breadcrumb, thus eliminating problem of overlapping.

What does the proposed API look like?

this.echartOption.series[0].breadcrumb.zIndex = 100