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:

[LFDanLu]

LGTM

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-canary
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

## API Changes

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