getSignInConfigForTenant(tenantId, providerIds = undefined) {
    const signInConfig = this.filterTenantConfig_(
        tenantId,
        UiHandlerConfig.SUPPORTED_SIGN_IN_CONFIG_KEYS);
    // If eligible providers are provided, only returns eligible providers in
    // the sign in options.
    const signInOptions = signInConfig['signInOptions'];
    if (signInOptions && providerIds) {
      const eligibleOptions = signInOptions.filter((option) => {
        if (typeof option === 'string') {
          return providerIds.includes(option);
        } else {
          return providerIds.includes(option['provider']);
        }
      });
      signInConfig['signInOptions'] = eligibleOptions;
    }
    return signInConfig;
  }
Returns the config object of the specific tenant for the sign-in flow.
@param {string} tenantId The tenant ID or project-level config key.
@param {?Array<string>=} providerIds The optional eligible provider IDs.
@return {!Object} The sign in UI config object for the tenant.