VictoryVoronoiContainer should allow default active points

This would allow users to show a chart with a point / tooltip auto-selected. The chart would still be interactive as usual while the user was hovering over it, but would have a default value otherwise.
This is currently possible in Victory with `externalEventMutations`, but it's complicated and fiddly.

Proposed API change:

A new prop called `defaultVoronoiValue` will be added. This prop should be given relative to data space (not svg coordinate space). The point(s) nearest to `defaultVoronoiValue` will be set `active` when the chart renders and `onMouseLeave`

```
defaultVoronoiValue: PropTypes.oneOfType([
PropTypes.number,
PropTypes.shape({
x: PropTypes.number,
y: PropTypes.number
})
])
```

Start with VictoryVoronoiContainer and inspect how externalEventMutations currently sets active points and how the container handles rendering and onMouseLeave. Trace the existing prop definitions and related interaction behavior before deciding where the new data-space value belongs. Done means defaultVoronoiValue selects the nearest point or points initially and after onMouseLeave while normal hover interaction remains unchanged.

Written by the indexing model from the issue text.