Handler exposes an external oauth provider flow (including the call back) as an oauth.handlers.AuthenticationHandler to allow our internal oauth server to use an external oauth provider for authentication.

Provider encapsulates the URLs, configuration, any custom authorize request parameters, and the method for transforming an access token into an identity, for an external OAuth provider.

RedirectorState combines state generation/verification with redirections on authentication success and error.

State handles generating and verifying the state parameter round-tripped to an external OAuth flow.