Store credentials, per provider.
environment is the global switch between sandbox and real money; each
provider's keys are selected from it, so a sandbox key in production (or the
reverse) is a configuration error rather than a silent test transaction.
These are namespaced away from Gamend.OAuth.Providers deliberately:
Apple issues different keys for Sign in with Apple and the App Store Server
API, from different portals, and the two used to collide on one name.