feat(fsutils): new utilities on top of io/fs.FS

[fredbi]

Adds OverlayFS, OpaqueFS, OsFS, GlobOsFS, MapFS (very much testing/fstest but not implying a test usage).

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.03%. Comparing base (606af28) to head (46e4e91).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   92.23%   93.03%   +0.80%     
==========================================
  Files          57       61       +4     
  Lines        2704     3015     +311     
==========================================
+ Hits         2494     2805     +311     
  Misses        166      166              
  Partials       44       44              

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