feature: add the ability to configure the disk widget's table columns

[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:

Adds the ability to specify the process widget's columns.

For example, specifying all valid columns:

[disk]
columns = ["disk", "mount", "used", "free", "total", "used%", "r/s", "w/s"]

Or:

[disk]
columns = ["disk", "mount", "w/s", "r/s", ]

Also a bunch of internal cleanups/refactoring.

Issue

If applicable, what issue does this address?

Closes: #1480

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

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

Attention: Patch coverage is 53.07263% with 84 lines in your changes missing coverage. Please review.

Project coverage is 41.97%. Comparing base (c8cba49) to head (75465ed).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/widgets/disk_table.rs	45.87%	59 Missing ⚠️
src/main.rs	0.00%	13 Missing ⚠️
src/data_conversion.rs	28.57%	10 Missing ⚠️
src/app/data_farmer.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1625       +/-   ##
===========================================
- Coverage   54.54%   41.97%   -12.58%     
===========================================
  Files         114      114               
  Lines       17621    17685       +64     
===========================================
- Hits         9612     7423     -2189     
- Misses       8009    10262     +2253     

Flag	Coverage Δ
macos-12	37.41% <53.07%> (+0.15%)	⬆️
ubuntu-latest	43.75% <53.07%> (-13.25%)	⬇️
windows-2019	37.32% <53.07%> (+0.15%)	⬆️

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