Adds a new key to the app and returns it with its secret. The secret is included in this response only and is never returned again — store it now. Use this together with revoke for zero-downtime rotation: add a key, roll it out across your clients, then revoke the old key. The public app key clients use to connect. The key secret, used for server-side request signing. Returned only when the key is created, and never shown again — store it securely. If lost, create a new key and revoke this one. When the key was revoked, or null if still active.