feat(lambda-at-edge): support external rewrites

[dphang]

Fixes: #832

[codecov]

Codecov Report

Merging #839 (dd6a39e) into master (ac28b2a) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #839      +/-   ##
==========================================
+ Coverage   80.33%   80.55%   +0.21%     
==========================================
  Files          63       63              
  Lines        2217     2242      +25     
  Branches      500      505       +5     
==========================================
+ Hits         1781     1806      +25     
  Misses        376      376              
  Partials       60       60              

Impacted Files	Coverage Δ
packages/libs/lambda-at-edge/src/api-handler.ts	88.88% <100.00%> (+0.88%)	⬆️
...ackages/libs/lambda-at-edge/src/default-handler.ts	95.27% <100.00%> (+0.07%)	⬆️
...ckages/libs/lambda-at-edge/src/routing/rewriter.ts	100.00% <100.00%> (ø)

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[dphang]

Not actually blacklisted but because the data returned is uncompressed (via fetch), copying this header tells CloudFront not to compress content, and then it fails at the browser since it cannot decompress already uncompressed content.