feat: add label style to segmented buttons

[rKatex]

Summary

Currently there is no way to change the style of the label of a SegmentedButton. This PR introduces the ability to give each element of the buttons-list an individual style.

{
    value: 'test',
    label: 'Test',
    style: {
        // Some wrapper style
    },
    labelStyle: {
        fontSize: 5,
    },
}

Test plan

Like in the example above, just apply some label style to a specific SegementedButton item.

[callstack-bot]

Hey @rKatex, thank you for your pull request 🤗. The documentation from this branch can be viewed here.