repo: replace "apt-key" with "gpg" to handle keys

[tigarmo]

The first commit adds some integration tests with some code that ends up getting removed in the second commit (a mock that is no longer needed). Reviewing per-commit shows the "story" but reviewing just the final result should be straighforward too.

CRAFT-1626

[codecov[bot]]

Codecov Report

Merging #29 (5669b66) into main (1c93776) will increase coverage by 0.03%. The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   78.39%   78.42%   +0.03%     
==========================================
  Files          11       11              
  Lines         486      496      +10     
  Branches      109      110       +1     
==========================================
+ Hits          381      389       +8     
- Misses         75       76       +1     
- Partials       30       31       +1     

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	95.18% <91.66%> (-2.08%)	:arrow_down:

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