Please make psutil an optional dependency for all platforms

[gilbahat]

Hi,

psutil is an optional project dependency (it's import-guarded for unsupported platforms like cygwin). psutil is a problematic package as it does not publish aarch64 wheels, which means that docker containers using it will have to contain a full build suite or dual build stages.

Given this nuisance and the fact that psutil is mostly immaterial to azure-cli, I would request that it be made an optional dependency.

[yonzhan]

@jiasli for awareness