add skeleton of `ConfiguredIdpOAuth` class

[tomholub]

• a new class called ConfiguredIdpOAuth that inherits from OAuth. It should contain new method called newAuthPopupForEnterpriseServerAuthenticationIfNeeded
• The method checks the storage for authentication and decides if a custom IdP is used or not. If custom IdP is not used, it just returns Promise<void>.
• else it shows a warning modal Custom IdP is configured on this domain, but it is not supported on browser extension yet. Authentication with Enterprise Server will continue using Google IdP until implemented in a future update.
• this new method should be called in newGoogleAcctAuthPromptThenAlertOrForward, right under await GlobalStore.acctEmailsAdd(response.acctEmail); inside the success case
• update the test from earlier issue add custom authentication config to local store #5317 to also expect this modal

[tomholub]

@martgil this is now ready to work on

[martgil]

Thanks, @tomholub! I'll start working on this as soon as I finish all my tasks for milestone 8.5.1.