[12.x]Feature: add sum method on Arr

[alipowerful7]

This PR introduces a new static method Arr::sum() to the Arr class.
It provides an easy way to calculate the sum of array values — including nested arrays —
and supports summing by a specific key or a custom callable (similar to Collection::sum()).

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!