Compound interrogative pronouns

In the given example, "whoever" (the compound interrogative pronoun) is used to emphasize the interrogative pronoun. "Who told you?" is simple question. but to put stress on "who", we use "whoever".

Your example should be written

Who ever told you so?

Here, "ever" is a separate word. As you point out, the question means "who told you so?" --- the word "ever" is not essential to the sense of the question; it just gives more emphasis.

So the word "whoever" is not in your example.

"Whoever" is not an interrogative word. It is called a "fused relative", and is used to like two clauses, as in these examples:

Whoever told you that was mistaken.

I would like to meet whoever told you that.