implement draft CachedContent handler

[jschlyter]

Work in progress, do not merge

[codecov-commenter]

Codecov Report

Merging #112 (079e687) into main (366d889) will decrease coverage by 0.04%.
The diff coverage is 82.94%.

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
- Coverage   77.59%   77.54%   -0.05%     
==========================================
  Files          41       43       +2     
  Lines        4342     4494     +152     
  Branches      841      863      +22     
==========================================
+ Hits         3369     3485     +116     
- Misses        709      729      +20     
- Partials      264      280      +16     

Impacted Files	Coverage Δ
src/cryptojwt/jwk/serialization.py	69.64% <69.64%> (ø)
src/cryptojwt/jwk/utils.py	88.88% <77.77%> (-11.12%)	⬇️
src/cryptojwt/cached_content.py	84.82% <84.82%> (ø)
src/cryptojwt/key_bundle.py	79.51% <97.29%> (-0.35%)	⬇️
src/cryptojwt/jwk/x509.py	59.32% <100.00%> (+1.42%)	⬆️
src/cryptojwt/key_issuer.py	80.25% <0.00%> (-0.95%)	⬇️

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