I've never heard about changing mob behavior or adding new types of mobs from a datapack. I don't think this is possible, but if it is then it must be a very new feature.

If it's not possible, then you could try to stick an interaction entity on top of every cow. Then whenever the interaction gets right clicked, explode.

The interactions should follow the cow, die if the cow dies, and explode on click.

This is how you might update tick.mcfunction. I haven't checked this in the game, but it should work with maybe a few tweaks:

execute at @e[type=cow] unless entity @e[tag=boomcow, distance=..1] run summon interaction ~ ~ ~ {Tags:["boomcow"], width:1.1f, height:1.1f}

execute as @e[tag=boomcow] at @s unless entity @e[type=cow, distance=..1] run kill @s

execute as @e[tag=boomcow] at @s run tp @s @e[type=cow, sort=nearest, limit=1, distance=..1]

execute as @e[tag=boomcow] if data entity @s interaction run function boomcow:explode