Duplicate a campaign via the API

What you'd be able to do: Create a new campaign (and its sheet) via the API by duplicating an existing one, instead of only being able to read campaigns and sheets.

Why it helps: Teams onboarding clients on a repeatable template need to spin up a new campaign programmatically. Cloning a known-good source campaign covers this without needing full sequence-builder access via API.

Today: Campaigns are read-only (GET) via the API, and sheets can only be created by uploading a file in the app.