Since it doesn't seem to be an option anywhere by default in Opera. Are there any addons available that can introduce the Ctrl+1 for first tab Ctrl+2 for second tab and so on behaviour that comes default in other browsers such as Chrome, Firefox, and Internet Explorer?
Not that I'm aware of. i urge it here in this add-on forum as a useful keyboard-shortcut feature for for an Opera add-on.
You might want to post a request for this type of keyboard functionality also in the Suggestion forum for an Opera feature. You could mention that all the other browsers provide that shortcut as part of the native browser, and it is useful.