Rename 'secret_key' to 'secret' in storage config

[svdf-org]

Fix: Correct field name for r2 storage_config in realtimekit

• The secret_key field throws error "storage_config.secret_key" is not allowed
• The correct field name is secret

Summary

Screenshots (optional)

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[roerohan]

This seems correct, verified in the API docs (screenshot below). @ravindra-cloudflare I'm approving this one, but you can take another look at it if you want.