Add plugins for fsspec caching and cache cleaning

[pnuu]

This PR adds the possibility to do caching with fsspec. Mostly useful for remotely read data that has a complicated structure which slows down the reading such as FCI L1c in S3 storage. Additional plugin is available for cleaning the cache.

• Tests added
• Tests passed
• Passes flake8 trollflow2
• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.87%. Comparing base (d6a96a9) to head (e7ae67c).
Report is 92 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   95.78%   95.87%   +0.08%     
==========================================
  Files          12       12              
  Lines        2989     3052      +63     
==========================================
+ Hits         2863     2926      +63     
  Misses        126      126              

Flag	Coverage Δ
unittests	95.87% <100.00%> (+0.08%)	⬆️

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[pnuu]

The failing test is due to a small change in Posttroll and the way we've implemented the tests (with mock). This shows up only in unstable tests because the change is not published.

[mraspaud]

Nice work! I'm wondering if this should really be its own plugin, or just an option to the reading plugin?

[mraspaud]

About the _fs...

[mraspaud]

LGTM!