add zfs io counters for linux and freebsd

[jamartin9]

Description

ZFS datasets in the disk menu lacked support for monitoring the read and written bytes.
This PR adds io-counters (read bytes and written bytes) for the named zfs datasets in the disks menu.

Screens:
Linux:

FreeBSD:

Testing

Tested on linux and freebsd, with and without zfs.

• Windows
• macOS
• Linux
• FreeBSD

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)

I did NOT test with an extensive zfs setup (no draid/raidz/spares) just my simple mirror pools (maybe use file backed pools to test?)

If any of the Issues should be addressed or a different approach used please let me know.

Feel free to close this PR if it is unwanted.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.51 🎉

Comparison is base (ccc7091) 31.84% compared to head (a6a2f80) 32.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1248      +/-   ##
==========================================
+ Coverage   31.84%   32.36%   +0.51%     
==========================================
  Files          97       94       -3     
  Lines       16581    16293     -288     
==========================================
- Hits         5281     5273       -8     
+ Misses      11300    11020     -280     

Flag	Coverage Δ
macos-12	33.45% <0.00%> (+0.01%)	⬆️
ubuntu-latest	33.29% <0.00%> (-0.16%)	⬇️
windows-2019	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/app/data_farmer.rs	9.28% <0.00%> (+0.16%)	⬆️
src/app/data_harvester/disks.rs	79.26% <ø> (ø)
...rc/app/data_harvester/disks/unix/linux/counters.rs	0.00% <0.00%> (ø)
src/app/data_harvester/disks/zfs_io_counters.rs	0.00% <0.00%> (ø)

... and 12 files with indirect coverage changes

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[ClementTsang]

Seems reasonable to add, I'll take a look in a bit.

[ClementTsang]

Mostly looks good, just a few small comments.

[jamartin9]

Thanks for the feedback! I have tried to address your comments. Please let me know if more is needed.