Interface RefreshHandlerClient

The one member of Google's OAuth2Client this package depends on.

Structural rather than imported: it keeps google-auth-library out of this package's dependencies, and keeps the client a server passes in type-identical to the one its own googleapis expects.

interface RefreshHandlerClient {
    refreshHandler?: () => Promise<
        { access_token: string; expiry_date: number },
    >;
}