well since oyu are looking for an addon to do this there are a number of bag addons that are just awesome to have for more than just your lock. ArkInventory and Baggins are the two that come to mind. The easiest of the two to configure would be ArkInventory but I prefer Baggins as it will show your 20 soul shards as one icon for instance which means when my bag is open it doesn't take up a huge portion of my screen. ArkInventory is handy because you can look at your alts bags as well as it has an option to add to an items tooltip of how many of that item you have on you, your bank and on your alts.
Anyways I would recommend ArkInventory if you want something fairly easy to set up and if you aren't very addon savvy and Baggins if you want to have your bag take up as less room as possible while open and don't mind diving into the options which can sometimes be a little hectic. Hope this helps!
