refactor: ⚡ hardcoded/preloaded default headers performance upgrade

[ixunio]

Performance upgrade: default/frequent encoded headers {"alg": "XXX", "typ": "JWT"} and {"alg": "XXX"} are hardcoded and preloaded in memory for each algorithm:

• ⚡ during encoding: use default preloaded encoded headers {"alg": "XXX", "typ": "JWT"} when no custom headers is specified, skipping validation
• ⚡ during decoding: if encoded headers is one of hardcoded, skip validation and instantly check for algorithm mismatch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.52%. Comparing base (4294fdc) to head (d0f161c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   99.49%   99.52%   +0.02%     
==========================================
  Files          10       10              
  Lines         993     1042      +49     
  Branches      118      122       +4     
==========================================
+ Hits          988     1037      +49     
  Partials        5        5              

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