[Lens] Choose legend position for both XY and pie charts separately

[wylieconlon]

Legend options are part of the chart-level options because there is only one legend shown. Legends should not be part of the Lens editor, because not every visualization has a legend- this decision is described below.

The most commonly used legend positions are Right and Bottom- having a bottom legend lets it have more space:

Decision: Legend settings are not shared across chart types

Because each chart type implements its state separately, I am not sure that we will be able to keep the legend position consistent when switching from XY -> Pie. For example, if the user chooses a Bottom legend for the XY chart, and then switches to Pie, I would expect that the legend becomes the default of Right.

Part of the styling meta issue

[elasticmachine]

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