Integrate revamped CallbackValidator

[dktapps]

Description

A couple of months ago, I rewrote CallbackValidator, with an improved API and support for stuff like union and intersection types.
https://github.com/pmmp/CallbackValidator/tree/rewrite

However, these improvements required some BC breaks.
This is a problem, because we have no clear documentation of which dependency libraries are considered API and which aren't.
This means that plugins could have used the legacy CallbackValidator API directly instead of Utils::validateCallableSignature().

We have 2 choices:

• Repackage CallbackValidator under a new name and namespace to allow it to coexist with the legacy CallbackValidator in 5.x
• Wait until 6.0 to integrate it

Alternative methods