support bz2 compression in avhrr_l1b_eps

[k3a]

EUMETCast distributes global METOP AVHRR data bz2-compressed (example AVHR_xxx_1B_M01_20241113130103Z_20241113130403Z_N_O_20241113133530Z.bz2).

This commit adds decompression support by reusing already-tested unzip_context helper function.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.17%. Comparing base (fd2cec6) to head (81daacd).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2978      +/-   ##
==========================================
+ Coverage   96.10%   96.17%   +0.07%     
==========================================
  Files         377      374       -3     
  Lines       55147    54977     -170     
==========================================
- Hits        52997    52873     -124     
+ Misses       2150     2104      -46     

Flag	Coverage Δ
behaviourtests	?
unittests	96.17% <100.00%> (-0.03%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 11824388461

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.207%

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Totals
Change from base Build 11815057382:	0.0%
Covered Lines:	53244
Relevant Lines:	55343

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💛 - Coveralls

[mraspaud]

Thanks a lot for this new functionality! Can you find a way to add a small unit test for this?