Export tar files, usually from a server

[richfitz]

This PR adds support for converting volumes into tar files, tar files into volumes, and extracting server-held copies of volumes into tar files.

Once this is sorted we can, alongside some thinning, export point-in-time snapshots from a server. This would also be useful for more manual backup and recovery.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8b3793c) 100.00% compared to head (6abaa4e) 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              main        #5    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           28        30     +2     
  Lines         1810      2060   +250     
  Branches       211       237    +26     
==========================================
+ Hits          1810      2060   +250     

Files	Coverage Δ
src/privateer2/cli.py	100.00% <100.00%> (ø)
src/privateer2/config.py	100.00% <100.00%> (ø)
src/privateer2/tar.py	100.00% <100.00%> (ø)
src/privateer2/util.py	100.00% <ø> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_config.py	100.00% <100.00%> (ø)
tests/test_tar.py	100.00% <100.00%> (ø)
tests/test_util.py	100.00% <ø> (ø)

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[r-ash]

This looks great