Add API feature item.
Return the feature item to add, or return false if existed.

@inner
@param {Object} logger logger the logger utilities
@param {Object} existed the added feature item map
@param {string} name the feature item name to add
@param {Object=} params the feature item params, optional
@return {Object|boolean}
function addFeatureItem(logger, existed, name, params) {
    if (!existed[name]) {
        existed[name] = true;
        let feature = {name};
        params && (feature.params = params);
        return feature;
    }

    logger.debug('duplicated feature declaration', name);
    return false;
}