FR (Primitives): Add Style Props for PasswordField button

This issue is related to the `amplify-ui@next` for React.

At the moment styling the show password button of a `PasswordField` component requires reaching out to CSS (at least as far as I could find/manage) and it would be really handy if it'd be possible to style the button in a similar way to the one available for the `descriptiveText` prop:
```js

}
/>
```
Alternatively it'd be great to even just be able to pass specific props to override the default ones like this:
```js

```

Start at the React PasswordField entry point and inspect how the existing descriptiveText prop and show-password button are handled. Determine which proposed API fits the component's existing primitives, then verify that the completed behavior supports styling the button with color, fontStyle, and fontSize.

Written by the indexing model from the issue text.