other: support hw.temperature-based temps on FreeBSD

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Add simple support for FreeBSD-based systems that may get temperatures using sysctl and hw.temperature.

Issue

If applicable, what issue does this address?

Closes: #797

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux
• FreeBSD

I tested whether the code works for some sysctl - I tried it with dev.cpu on my FreeBSD box, and it works. I did not test it for hw.temperature, as that does nothing for my x86-64-based FreeBSD machine.

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Base: 21.31% // Head: 21.55% // Increases project coverage by +0.23% 🎉

Coverage data is based on head (a8b1340) compared to base (f89b243).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1024      +/-   ##
==========================================
+ Coverage   21.31%   21.55%   +0.23%     
==========================================
  Files          75       75              
  Lines       14754    14590     -164     
==========================================
  Hits         3145     3145              
+ Misses      11609    11445     -164     

Impacted Files	Coverage Δ
src/bin/main.rs	36.40% <0.00%> (-0.18%)	⬇️
src/canvas.rs	0.00% <0.00%> (ø)
src/app/data_farmer.rs	0.00% <0.00%> (ø)
src/app/data_harvester.rs	0.00% <0.00%> (ø)
src/canvas/widgets/mem_basic.rs	0.00% <0.00%> (ø)
src/canvas/widgets/mem_graph.rs	0.00% <0.00%> (ø)
src/app/data_harvester/memory/general/heim.rs	0.00% <0.00%> (ø)
src/data_conversion.rs	14.22% <0.00%> (+1.50%)	⬆️

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