After digging in the Wiki Quest Page's source code, I was able to easily locate the optional quest tag.
Sadly the quests in Act 3 have yet to receive proper documentation on the wiki and I'm not that far in the game to build upon said documentation myself.
So here are the quests that have been flagged as "optional" by the PoE community. It's worth noting that Deal with the Bandits has not been documented as optional, perhaps we have a different definition of side quests. Listed below will be the quests that you don't need to complete in order to move forward in the game in Act 1 & 2.
Here's hoping this will get you to complete every quests in the game ;)
Additional sources that could yield information in the future (if updated) :