Create a cloud design

Scope: write. The design must validate. Send an Idempotency-Key header to make retries safe: the first response is replayed for 24 hours (Idempotent-Replayed: true). Creation through the API stops once the account's personal designs (or a team key's team) number 2000, counting designs saved from the editor too (409 DESIGN_LIMIT_REACHED), and design writes — create, update, delete, share — are limited to 30 a minute per account across all its keys.