Expose the reason parameter in the flag endpoint

[testableapple]

🔗 Issue Links

Resolve https://github.com/GetStream/ios-issues-tracking/issues/801

🎯 Goal

Expose the reason parameter in the flag endpoint

🧪 Manual Testing Notes

N/A

☑️ Contributor Checklist

• [x] I have signed the Stream CLA (required)
• [x] This change follows zero ⚠️ policy (required)
• [x] This change should be manually QAed
• [x] Changelog is updated with client-facing changes
• [ ] New code is covered by unit tests
• [ ] Comparison screenshots added for visual changes
• [ ] Affected documentation updated (docusaurus, tutorial, CMS)

🎁 Meme