[Lens] Pie and donuts should have a size ratio setting

[wylieconlon]

The default pie and donut charts in Lens use 100% of the smallest dimension in their area, which is usually height. This prevents external labels from rendering above or below the charts. One of the chart options that could be set by users is size ratio, something like this:

• Large: 100%
• Medium: 80%
• Small: 60%

We could also have a setting for Donut charts specifically, since the default inner ratio is 30%. Users may want to have different-looking donut charts.

This was feedback from @markov00

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

This could also solve the problem users are having when they change over from legacy visualizations to elastic-charts....they see donuts with a smaller hole:
Before:

After:

[ghudgins]

there is also alignment issue in the above screenshots - need to probably file a separate charts issue as it relates to consistency