feature: Use ps as fallback to query CPU usage under macOS (#380)

[Frederick888]

Description

When running without elevated permissions under macOS, sysinfo cannot
query states of processes by root user, which results in 0.0% CPU usage
for all this kind of processes (and state = Unknown).

Here we use ps, which has SUID, as a fallback to query CPU usages.
This can be potentially applied to other properties if needed in the
future (we'll need a proper struct and parser).

Issue

Closes: #380

Type of change

• New feature (non-breaking change which adds functionality)

Test methodology

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

Checklist

• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
• Passes CI pipeline (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

I admit this is quite hacky and I'm not sure if it's worth the effort maintaining, but unfortunately users are not always able to run btm with elevated permissions and this has come quite helpful to myself.

[ClementTsang]

Sorry for the late response, been a bit busy recently - I'll take a look within a day or two of the PR.

[ClementTsang]

Sorry for taking so long, LGTM and works fine in my macOS environment.

@all-contributors add @Frederick888 for code

[allcontributors]

@ClementTsang

I've put up a pull request to add @Frederick888! 🎉