Allow host apps to short-circuit account syncing

[ctarda]

After a login is completed the delegate method sync(credentials: AuthenticatorCredentials, onCompletion: @escaping () -> Void) is called to allow host apps to sync the account.

We want host apps to be able to insert custom UI instead of just complete the login process. For example, we might want WooCommerce to check that the url entered when logging with store address belongs to the account the user logged in to, and present UI to warn toe user of that mismatch.

[ctarda]

It turns out this can be achieved, at least on WooCommerce, by waiting until the delegate method to launch the login epilogue is called. So closing for now.