refactor: optimize username cloning for Unix processes

[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:

This tries to reduce the number of string clones when collecting usernames, by storing them in a Option<Arc<str>> up until we display them.

This also pulls in some refactors from #1807.

Issue

If applicable, what issue does this address?

Closes: #

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

❌ Patch coverage is 11.53846% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.48%. Comparing base (c8614bf) to head (aaa69b4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/widgets/process_table/process_data.rs	9.09%	10 Missing ⚠️
src/widgets/process_table/process_columns.rs	0.00%	4 Missing ⚠️
src/collection/processes/unix/user_table.rs	0.00%	3 Missing ⚠️
src/widgets/process_table/query.rs	0.00%	3 Missing ⚠️
src/collection/processes/linux/mod.rs	0.00%	1 Missing ⚠️
src/collection/processes/unix/process_ext.rs	0.00%	1 Missing ⚠️
src/collection/processes/windows.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1816      +/-   ##
==========================================
- Coverage   43.53%   43.48%   -0.05%     
==========================================
  Files         115      115              
  Lines       16302    16297       -5     
==========================================
- Hits         7097     7087      -10     
- Misses       9205     9210       +5     

Flag	Coverage Δ
macos-14	37.34% <8.33%> (-0.01%)	⬇️
ubuntu-latest	45.08% <12.50%> (-0.08%)	⬇️
windows-2022	37.68% <9.52%> (-0.03%)	⬇️

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