Support archive type detection for .tgz annex keys

[mih]

This seems to be at least as sensible as .tar.

Closes #517

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1f7d9f5) 92.43% compared to head (202fec4) 92.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   92.43%   92.46%   +0.03%     
==========================================
  Files         128      128              
  Lines        9606     9612       +6     
  Branches     1040     1042       +2     
==========================================
+ Hits         8879     8888       +9     
+ Misses        703      700       -3     
  Partials       24       24              

Files	Coverage Δ
datalad_next/types/archivist.py	100.00% <100.00%> (ø)
datalad_next/types/tests/test_archivist.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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