add optional paths to find_executable function

[nebukadhezer]

Changelog Description

In order to use find executable in a broader scope, I added an optional paths kwarg to extend the actual search paths

Additional info

Testing notes:

[iLLiCiTiT]

Hello, may I ask about the use-case?

[nebukadhezer]

I use the function to look for executables in an env that is not os.environ.
this is used here ynput/ayon-applications#124
Rez is used to resolve an env in a prelaunch hook and then we search for the executable in the launch_context.env which was modified by the hook. I hope that explains it.