I'd like to configure Graphic settings through addons such as texture quality, water reflection, distance, stuff like that. However, I couldn't find such API in reference. Could you tell me any API which allows you control Graphic settings if exists?
There is no API references for those. They are cVars in-game. cVars are not document as such but the UserSettings.txt has ones you are interested in. Poking at cVars can crash the client and make unplayable if one is set wrong. There is an add-on called ESOToolbox that is making some changes to user settings via cVars.