feat: add contentful provider

[ykzts]

Resolve #330

It uses Contentful's Images API to transform images.

[pi0]

Thanks!

[codecov-commenter]

Codecov Report

Merging #398 (f0b1154) into main (026be95) will increase coverage by 0.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
+ Coverage   59.32%   59.90%   +0.58%     
==========================================
  Files          27       28       +1     
  Lines         622      631       +9     
  Branches      196      189       -7     
==========================================
+ Hits          369      378       +9     
  Misses        253      253              

Impacted Files	Coverage Δ
src/provider.ts	74.28% <ø> (ø)
src/runtime/providers/contentful.ts	100.00% <100.00%> (ø)

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