Get the managed signing key a new release build of the app embeds.

Returns the managed patch-signing key a new release build of the app embeds: the app's own key if it has one, else its organization's current key. The organization's key is created on the first call, so repeated calls return the same key until it is rotated. The CLI calls this before it builds each platform of a release, embeds the public key, and reports the key's id with each release artifact it uploads. Requires permission to create the app's releases.