NewOAuthConfig creates a new `oauth.Config` for the provided `AuthProvider` based on the built in auth provider configs.

WithEmulatorHost specifies the authentication emulator host to use in testing.

WithUserCache provides a user cache for the authentication client.

DefaultUser represents the default user to be loaded and stored in the `UserCache`.

GoogleAuthProvider provides support for authenticating with Google.

DefaultApp is the default, built in, firebase application.

App represents a firebase application.

Auth handles authenticating users.

User contains details of a user from Firebase.

AuthOption is a generic interface used to apply options when creating a new authentication client.

UserCache provides an interface for caching authenticated users and their `RefreshToken`.

AuthProvider is an enumeration of supported authentication providers.

MemoryUserCache implements the `UserCache` interface with an in-memory map.