feat: walk_dir function (finds all files in sub directories)

[ali77gh]

Motivation

I was working on my personal project and using Tokio as my runtime. I needed to walk through directories recursively, find files, and perform other tasks. I struggled a lot to accomplish this with Tokio, which made me wonder why there is no walk function in Tokio's file system module. It seems like a lightweight and useful feature to have.
Python, for instance, has os.walk in its standard library.

Solution

With all that said, I added the walk_dir() function to tokio::fs to make it easier for others to search over files recursively
using Tokio.

If there is any type of problem in this pull request. Just let me know.

[Darksonn]

I don't want to add this because there is no std::fs equivalent.