Add inputRef support for RAC SliderThumb

[yihuiliao]

Currently there's no way to pass a inputRef to RAC SliderThumb since it's constructed internally but we'd like to be able to for 🌈

✅ Pull Request Checklist:

• [ ] Included link to corresponding React Spectrum GitHub Issue.
• [ ] Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• [ ] Filled out test instructions.
• [ ] Updated documentation (if it already exists for this component).
• [ ] Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project: