refactor: simplify algorithm instances cache

[ixunio]

Given we don't have cryptography as an optional dependency, we can now store instances directly as globals. Also, SUPPORTED_ALGORITHMS was renamed VALID_ALGORITHMS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.61%. Comparing base (654bb78) to head (fbb1701).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   99.52%   99.61%   +0.09%     
==========================================
  Files          10       10              
  Lines        1042     1030      -12     
  Branches      122      118       -4     
==========================================
- Hits         1037     1026      -11     
+ Partials        5        4       -1     

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