I need help in adding attack effect (collision based particle systems) to all melee weapons, are guys exposing any API for adding such events?
You mean a weapon trail? Yea, the asset that you pulled that pic from has an API for it as well. Though all you really need to do is put a start and a stop point on each weapon and call it either in the input or on the animator (enable/disable or any custom API). You can also use the Event OnHit if you only only want it to appear when you hit something, or onEnable damage to turn on the weapon trail when your collider is enabled on your weapon (start/stop damage on states in the animator)