[radio group] Allow to be used standalone without RadioGroup

# Feature request

## Summary

Enhance the `Radio` component to support standalone usage without requiring it to be part of a `RadioGroup`. Specifically, add support for `checked` and `onChange` props. This will allow developers to manage the state of individual `Radio` components, enabling more flexibility for custom visual designs or use cases where `RadioGroup` is not needed.

## Examples in other libraries

- [`Material-UI Radio`](https://mui.com/material-ui/react-radio-button/#standalone-radio-buttons)
- [`Ant Design Radio`](https://ant.design/components/radio/#components-radio-demo-basic)

## Motivation

The current implementation tightly couples the `Radio` component with `RadioGroup`, making it less versatile in scenarios where developers:
1. Need a static representation of radio buttons for read-only or decorative purposes.
2. Want to use the `Radio` component for custom designs without needing full `RadioGroup` functionality.
3. Handle state or interactions independently for each `Radio`, such as in the case of custom forms, polls, or conditional UI.

By allowing `Radio` to accept `checked` and `onChange`, developers can use it more flexibly in a variety of contexts. Currently, developers building their own components on Base UI need to create two versions of `Radio` components: one based on the Base UI `Radio` for use with `RadioGroup`, and another that is a clone of the first to be used independently without `RadioGroup`. Supporting `checked` and `onChange` directly in the Base UI `Radio` would eliminate the need for such duplication, simplifying development and improving consistency across implementations.