[RAC] Export HeaderContext

[hosmelq]

This PR only exports the HeaderContext.

I want to pass props through context to the Header component, but it is not possible right now.

✅ 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:

[reidbarber]

Thanks!