Add documents to a collection

Required, at most 200 bytes. A replay with the same key, the same files (names, exact bytes, in order) and the same plan returns the version the key claimed, in whatever state it has reached; the same key with a different file set or plan is 409 {"error":{"type":"conflict","code":"idempotency_key_reuse","message":"this key was used for different input"}}. Operations on one source settle one at a time: a new request while another is pending is 409 source_update_in_progress.