Client needs ability to set batch expiration date, for multiple key values. Existing Client Method: ``` Task<bool> StringSetAsync(KeyValuePair<RedisKey, RedisValue>[] values, When when = When.Always, CommandFlags flags = CommandFlags.None); ``` Needed Client Method: ``` Task<bool> StringSetAsync(KeyValuePair<RedisKey, RedisValue>[] values, TimeSpan? expiry, When when = When.Always, CommandFlags flags = CommandFlags.None); ```