add custom authentication config to local store

[tomholub]

• A new type should be created DomainConfiguration = { authentication: clientConfigurationJson: ClientConfigurationJson } and also AuthenticationConfiguration = { oauth: { clientId: string, clientSecret: string?, redirectUrl: string, authCodeUrl: string, tokensUrl: string, } }.
• ExternalService.fetchAndSaveClientConfiguration to be updated: It should first call /api/v1/client-configuration/authentication?domain=${domain} (NOT authenticated, add a parameter to request), and save the whole API response (which can be empty on default deployments, or contain the {oauth: ...} type) into storage with await AcctStore.set(this.acctEmail, { authentication: r });. And after that proceed with the rest of the method
• Update FES mock by adding this /authenticate endpoint, which always returns an empty json, except when otherwise configured for a particular test.
• Add a test which will have some specific made up values configured in the FES mock, and verify these values from extension settings page -> local_store link on the bottom, which should newly also display the authentication contents