Allow centering the FAB component

### Description

The [fab](https://docs.nativebase.io/fab) component only accepts the following values for its `placement` prop:
"top-right" | "top-left" | "bottom-right" | "bottom-left"

I tried extending the `placementProps` doing something like:
```'top-center': { top: '50%', left: '50%', position: 'absolute', transform: 'translate(-50%, -50%)' },```
But it is not perfectly centered.

It would be great to be able to add a `center` value for the FAB `placement` prop

### Problem Statement

It does not seem possible to center a FAB element using this library

### Proposed Solution or API

Add an additional `center` value to the placementProps of the FAB component.

### Alternatives

_No response_

### Additional Information

_No response_