Add support for percentage on borderRadius ViewStyle props (#44140)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44140

Why?
Previously we didn't support using percentages like:
```
style={{
  width=100,
  height=100,
  borderRadius='100%',
}}
```

These percentages refer to the corresponding dimension of the border box.

What?
Change the unit type for `BorderRadii` values to `ValueUnit`. This type allows us to have an object containing a `float`, and a `UnitType` properties. With this we conditionally calculate the corresponding point (dp) value for a given percentage (considering size). Ex:

```
result = {raw_percentage_value} / 100 * (max(height, width))
```

We know the maximum border radius for our current implementation is half the dp of the shorter side of our view, hence why we consider half our maximum view side as equivalent to 100%.

Note: We still don't support vertical/horizontal border radii

## Changelog:

[iOS][Added] - Added support for using percentages when defining border radius related properties.

Reviewed By: NickGerleman

Differential Revision: D56198302

fbshipit-source-id: 6cd510b1c7164dcb82ca5ad8a9861c5ce5c8b15b