Heatmap: Exposes the positioning of the heatmap cells/containe

[qn895]

Is your feature request related to a problem? Please describe.
While trying to add extra functionality to a component that consumes the heatmap, we found that knowing the exact position of where the heatmap ends is fairly tricky.

Looks like the heatmap does some auto calculation of the cell width by taking into account several things (like the x axis labels, padding, number of buckets, etc.)

Describe the solution you'd like
It would be good to have a way to expose out the dimensions of the actual heatmap, after taking into account the axis and the labels. A possible solution would be to have something similar to onResize or onRenderChange callback that provides the calculated dimensions.

Kibana Cross Issues
elastic/kibana#69538

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present