App state interface

[4javier]

This little PR add two short interfaces to type the appState that can be passed to login command to be stored before redirect.
At the moment the only typed parameter is optional target but other ones could be added in the future, and custom ones are allowed by interface signature.

This way, when writing a loginWithRedirect(...) call, both appState object both its string target property will be suggested by autocompletion.

To get this working two interfaces have been created

/**
 * Extends auth0-spa-js RedirectLoginOptions narrowing
 * appState property type to NgAppState
 */
export interface NgRedirectLoginOptions extends RedirectLoginOptions {
  appState?: NgAppState;
}

and

/**
 * Angular specific state to be stored before redirect
 */
export interface NgAppState {
  /**
   * Target path the app gets routed to after
   * callback from login (defaults to '/')
   */
  target?: string;

  /**
   * For any custom parameter the user would store in appState
   */
  [key: string]: any;
}

With this setup, future angular specific options can be kept separated from auth0-spa-js code without interfering with it, since the assertions it issues on the extended RedirectLoginOptions only care about it being an object and about its basic properties types.

[frederikprijck]

Thanks for the PR. We believe having a typed appState is an improvement, however we want to make a change to our Auth0-SPA-JS SDK to ensure other wrapping SDKs (Angular, React, ...) benefit from that same typed AppState.

Once auth0/auth0-spa-js#846 is merged and released, we should be able to update the Angular SDK by mostly adding some generic types to AuthService, something along the lines of:

export interface NgAppState {
  target?: string;
  [key: string]: any;
}

export class AuthService<TAppState extends NgAppState = NgAppState> implements OnDestroy {
  private appStateSubject$ = new ReplaySubject<TAppState>(1);

  loginWithRedirect(options?: RedirectLoginOptions<TAppState>): Observable<void> {
    return from(this.auth0Client.loginWithRedirect(options));
  }
  
  handleRedirectCallback(url?: string): Observable<RedirectLoginResult<TAppState>> {
    return defer(() => this.auth0Client.handleRedirectCallback<TAppState>(url)).pipe(
      withLatestFrom(this.authState.isLoading$),
      tap(([result, isLoading]) => {
        if (!isLoading) {
          this.authState.refresh();
        }
        const appState = result?.appState;
        const target = appState?.target ?? '/';

        if (appState) {
          this.appStateSubject$.next(appState);
        }

        this.navigator.navigateByUrl(target);
      }),
      map(([result]) => result)
    );
  }
}