Victorybar should have an option for vertically aligning the VictoryLabel within each bar

# Feature Requests

### Checklist

- [x] I've read through the [Docs](https://formidable.com/open-source/victory/docs) and [Guides](https://formidable.com/open-source/victory/guides) to make sure this functionality doesn't already exist

- [x] I've searched open issues to make sure I'm not opening a duplicate issue

### Description

`VictoryBar` doesnt offer us a way to vertically align the text as part of the bar.
`VictoryLabel` provides a `dy` but we have no means to compute half the size of the bar