keys: per-repository keyrings

[lengau]

CRAFT-1627

• [x] Have you followed the guidelines for contributing?
• [x] Have you signed the CLA?
• [x] Have you successfully run tox?

---

[codecov[bot]]

Codecov Report

Merging #32 (dd99e00) into main (7bf8c35) will increase coverage by 2.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   78.42%   80.57%   +2.14%     
==========================================
  Files          11       11              
  Lines         496      520      +24     
  Branches      110      114       +4     
==========================================
+ Hits          389      419      +30     
+ Misses         76       73       -3     
+ Partials       31       28       -3     

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	97.87% <100.00%> (+2.69%)	:arrow_up:
craft_archives/repo/apt_sources_manager.py	93.02% <100.00%> (+6.01%)	:arrow_up:
craft_archives/repo/errors.py	80.64% <100.00%> (+2.86%)	:arrow_up:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[lengau]

Lint issue will be fixed in another PR (it's a timing issue as a result of the ruff update PR and the apt-keys PR being merged around the same time). Coverage I will make more commits for better tests, but wanted to get this up for review to start