Foxes are known to eat eggs as a fun snack for them. I feel foxes should be able to eat eggs by default...
You can make the foxes eat eggs by adding the "food" component to the item

I tried adding eggs as a food source for the fox using datapacks by adding "fox_food.json" into "minecraft/tags/items/": 

{
    "values": [
        "minecraft:sweet_berries",
        "minecraft:glow_berries",
        "minecraft:egg",
        "minecraft:turtle_egg"
    ]
} 

But then I found out that this file controls the breeding items....

Food component: 

/give @s egg[food={nutrition:1,saturation:1}]