Doesn't the lid have a spout? The least expensive way would be to replace the lid with one that had a spout. Just whack a new lid on the container and pour away.
I've seen horizontal valves - but that would mean replacing the container.
Not usually something you see in an ordinary 5 gallon pail.
I wonder if this is what you are looking for: Drum Faucet, Plastic Drum Faucets in Stock - ULINE
The 3/4 inch one is what I am thinking of. I have seen these before, I just am not sure where, perhaps janitorial supplies. The bucket had a two part cap, the larger cap would be removed for the pump that was posted. In that cap was a smaller plug which was the same thread as a faucet for a garden hose. The faucets I have seen were always a bronzy color plastic.
I'm not sure why they show this with the spout on the bottom. When you pour something from a can or a bucket the spout should be on top so you do not get the "glug glug" as air is sucked into the can.